OpenSSL doesn't initialize bytes on purpose in order to generate
additional entropy. Valgrind isn't too happy about that though, causing
it to generate warninings about various issues regarding use of
uninitialized bytes.

We traditionally had some infrastructure to silence these errors in our
OpenSSL stream implementation, where we invoke the Valgrind macro
`VALGRIND_MAKE_MEMDEFINED` in various callbacks that we provide to
OpenSSL. Naturally, we only include these instructions if a preprocessor
define "VALGRIND" is set, and that in turn is only set if passing
"-DVALGRIND" to CMake. We do that in our usual Azure pipelines, but we
in fact forgot to do this in our nightly build. As a result, we get a
slew of warnings for these nightly builds, but not for our normal
builds.

To fix this, we could just add "-DVALGRIND" to our nightly builds. But
starting with commit d827b11b (tests: execute leak checker via CTest
directly, 2019-06-28), we do have a secondary variable that directs
whether we want to use memory sanitizers for our builds. As such, every
user wishing to use Valgrind for our tests needs to pass both options
"VALGRIND" and "USE_LEAK_CHECKER", which is cumbersome and error prone,
as can be seen by our own builds.

Instead, let's consolidate this into a single option, removing the old
"-DVALGRIND" one. Instead, let's just add the preprocessor directive if
USE_LEAK_CHECKER equals "valgrind" and remove "-DVALGRIND" from our own
pipelines.

azure: fix misleading messages printed to stderr being

tests: iterator: fix iterator expecting too few items

The testcase iterator::workdir::filesystem_gunk sets up quite a lot of
directories, which is why it only runs in case GITTEST_INVASIVE_SPEED is
set in the environment. Because we do not run our default CI with this
variable, we didn't notice commit 852c83ee (refs: refuse to delete
HEAD, 2020-01-15) breaking the test as it introduced a new reference to
the "testrepo" repository.

Fix the oversight by increasing the number of expected iterator items.

In order to properly tear down the test environment, we will kill
git-daemon(1) if we've exercised it. As git-daemon(1) is spawned as a
background process, it is still owned by the shell and thus killing it
later on will print a termination message to the shell's stderr, causing
Azure to report it as an error.

Fix this by disowning the background process.

The Docker entrypoint currently creates the libgit2 user with "useradd
--create-home". As we start the Docker container with two volumes
pointing into "/home/libgit2/", the home directory will already exist.
While useradd(1) copes with this just fine, it will print error messages
to stderr which end up as failures in our Azure pipelines.

Fix this by simply removing the "--create-home" parameter.

Without the "--silent" parameter, curl will print a progress meter to
stderr. Azure has the nice feature of interpreting any output to stderr
as errors with a big red warning towards the end of the build. Let's
thus silence curl to not generate any misleading messages.

When building dependencies for our Docker images, we first download the
sources to disk first, unpack them and finally remove the archive again.
This can be sped up by piping the downloading archive into tar(1)
directly to parallelize both tasks. Furthermore, let's silence curl(1)
to not print to status information to stderr, which tends to be
interpreted as errors by Azure Pipelines.

transports: http: fix custom headers not being applied

azure: fix Coverity pipeline

In commit b9c5b15a (http: use the new httpclient, 2019-12-22), the HTTP
code got refactored to extract a generic HTTP client that operates
independently of the Git protocol. Part of refactoring was the creation
of a new `git_http_request` struct that encapsulates the generation of
requests. Our Git-specific HTTP transport was converted to use that in
`generate_request`, but during the process we forgot to set up custom
headers for the `git_http_request` and as a result we do not send out
these headers anymore.

Fix the issue by correctly setting up the request's custom headers and
add a test to verify we correctly send them.

There's several issues with our Coverity builds, like e.g. missing wget
in our containers. Simplify our Coverity pipeline and fix these issues.

Back in commit 5a6740e7 (azure: build Docker images as part of the
pipeline, 2019-08-02), we have converted our pipelines to use self-built
Docker images to ease making changes to our Dockerfiles. The commit
didn't adjust our Coverity pipeline, though, so let's do this now.

In commit bbc0b20b (azure: fix Coverity's build due to wrong container
name, 2019-08-02), Coverity builds were fixed to use the correct
container names. Unfortunately, the "fix" completely broke our Coverity
builds due to using wrong syntax for the Docker task. Let's fix this by
using "imageName" instead of the Docker dict.

azure: tests: re-run flaky proxy tests

While we already do have logic to re-run flaky tests, the FAILED
variable currently does not get reset to "0". As a result, successful
reruns will still cause the test to be registered as failed.

Fix this by resetting the variable accordingly.

The proxy tests regularly fail in our CI environment. Unfortunately,
this is expected due to the network layer. Thus, let's re-try the proxy
tests up to five times in case they fail.

fetchhead: strip credentials from remote URL

If fetching from an anonymous remote via its URL, then the URL gets
written into the FETCH_HEAD reference. This is mainly done to give
valuable context to some commands, like for example git-merge(1), which
will put the URL into the generated MERGE_MSG. As a result, what gets
written into FETCH_HEAD may become public in some cases. This is
especially important considering that URLs may contain credentials, e.g.
when cloning 'https://foo:bar@example.com/repo' we persist the complete
URL into FETCH_HEAD and put it without any kind of sanitization into the
MERGE_MSG. This is obviously bad, as your login data has now just leaked
as soon as you do git-push(1).

When writing the URL into FETCH_HEAD, upstream git does strip
credentials first. Let's do the same by trying to parse the remote URL
as a "real" URL, removing any credentials and then re-formatting the
URL. In case this fails, e.g. when it's a file path or not a valid URL,
we just fall back to using the URL as-is without any sanitization. Add
tests to verify our behaviour.

azure-pipelines: properly expand negotiate passwords

To allow testing against a Kerberos instance, we have added variables
for the Kerberos password to allow authentication against LIBGIT2.ORG in
commit e5fb5fe5 (ci: perform SPNEGO tests, 2019-10-20). To set up the
password, we assign

    "GITTEST_NEGOTIATE_PASSWORD=$(GITTEST_NEGOTIATE_PASSWORD)"

in the environmentVariables section which is then passed through to a
template. As the template does build-time expansion of the environment
variables, it will expand the above line verbosely, and due to the
envVar section not doing any further expansion the password variable
will end up with the value "$(GITTEST_NEGOTIATE_PASSWORD)" in the
container's environment.

Fix this fixed by doing expansion of GITTEST_NEGOTIATE_PASSWORD at
build-time, as well.

cred: change enum to git_credential_t and GIT_CREDENTIAL_*

Update link to libgit2 Julia language binding

We avoid abbreviations where possible; rename git_cred to
git_credential.

In addition, we have standardized on a trailing `_t` for enum types,
instead of using "type" in the name.  So `git_credtype_t` has become
`git_credential_t` and its members have become `GIT_CREDENTIAL` instead
of `GIT_CREDTYPE`.

Finally, the source and header files have been renamed to `credential`
instead of `cred`.

Keep previous name and values as deprecated, and include the new header
files from the previous ones.

Return int from non-free functions

Stop returning a void for functions, future-proofing them to allow them
to fail.

Stop returning a void for functions, future-proofing them to allow them
to fail.

Stop returning a void for functions, future-proofing them to allow them
to fail.

Stop returning a void for functions, future-proofing them to allow them
to fail.

Stop returning a void for functions, future-proofing them to allow them
to fail.

Stop returning a void for functions, future-proofing them to allow them
to fail.

Stop returning a void for functions, future-proofing them to allow them
to fail.

Stop returning a void for functions, future-proofing them to allow them
to fail.

Stop returning a void for functions, future-proofing them to allow them
to fail.

Stop returning a void for functions, future-proofing them to allow them
to fail.

HTTP: Support Apache-based servers with Negotiate

Disambiguate between general network problems and HTTP problems in error
codes.

Download poxygit, a debugging git server, and clone from it using NTLM,
both IIS-style (with connection affinity) and Apache-style ("broken",
requiring constant reauthentication).