Tracing is meant to be extremely low-impact when not enabled.  We
currently ship no tracing calls in libgit2, but if / when we do, the
tracing infrastructure is created to skip tracing as quickly as
possible.  It should compile to a simple test when tracing is off.

Thus, there's on reason to not enable it by default.

CI Build Updates

Provide usage hints to valgrind.  We trust the data coming back from
OpenSSL to have been properly initialized.  (And if it has not, it's an
OpenSSL bug, not a libgit2 bug.)

We previously took the `VALGRIND` option to CMake as a hint to disable
mmap.  Remove that; it's broken.  Now use it to pass on the `VALGRIND`
definition so that sources can provide valgrind hints.

valgrind will warn that OpenSSL will use undefined data in connect/read
when talking to certain other TLS stacks.  Thankfully, this only seems
to occur when gcc is the compiler, so hopefully valgrind is just
misunderstanding an optimization.  Regardless, suppress this warning.

We currently talk to Azure Repos for executing an online test
(online::clone::path_whitespace).  Add a simpler test to talk to Azure
Repos to make it obvious that strange test failures are not likely the
whitespace in the path, but actually a function of talking to Azure
Repos itself.

Our docker builds are getting expensive, let's cache some of this.

The valgrind in the PPA is broken and ignores `--exit-errorcode`.
Build and install our own.

Use a multi-stage docker build so that we can cache early stages and not
need to download the apt-provided dependencies during every build (when
only later stages change).

Deleting the apt cache can be helpful for reducing the size of a
container, but since we don't push it anywhere, it only hinders our
ability to debug problems while working on the container.  Keep it.

libcrypto will read uninitialized memory as entropy.  Suppress warnings
from this behavior.

The lsb-release command is missing on our images; just show the
information from the file instead of relying on it.

Provide a sane default for `CMAKE_GENERATOR` in the build script so that
it can be invoked without having to set that in the environment.

patch_parse: use paths from "---"/"+++" lines for binary patches

Follow 308 redirect in WinHTTP transport

fileops: correct error return on p_lstat failures when mkdir

For some patches, it is not possible to derive the old and new file
paths from the patch header's first line, most importantly when they
contain spaces. In such a case, we derive both paths from the "---" and
"+++" lines, which allow for non-ambiguous parsing. We fail to use these
paths when parsing binary patches without data, though, as we always
expect the header paths to be filled in.

Fix this by using the "---"/"+++" paths by default and only fall back to
header paths if they aren't set. If neither of those paths are set, we
just return an error. Add two tests to verify this behaviour, one of
which would have previously caused a segfault.

IIRC I got a strange return once from lstat, which translated in a weird
error class/message being reported. As a safety measure, enforce a -1 return in
that case.

config_mem: implement support for snapshots

patch_parse: fix segfault when header path contains whitespace only

Similar as in commit dadbb33b (Fix crash if snapshotting a
config_snapshot, 2019-11-01), let's implement snapshots for in-memory
configuration entries. As this deletes more code than it adds, it
doesn't make any sense to not allow for this and allows users to treat
config backends mostly the same.

When parsing header paths from a patch, we reject any patches with empty
paths as malformed patches. We perform the check whether a path is empty
before sanitizing it, though, which may lead to a path becoming empty
after the check, e.g. if we have trimmed whitespace. This may lead to a
segfault later when any part of our patching logic actually references
such a path, which may then be a `NULL` pointer.

Fix the issue by performing the check after sanitizing. Add tests to
catch the issue as they would have produced a segfault previosuly.

config_file: fix race when creating an iterator

When creating a configuration file iterator, then we first refresh the
backend and then afterwards duplicate all refreshed configuration
entries into the iterator in order to avoid seeing any concurrent
modifications of the entries while iterating. The duplication of entries
is not guarded, though, as we do not increase the refcount of the
entries that we duplicate right now. This opens us up for a race, as
another thread may concurrently refresh the repository configuration and
thus swap out the current set of entries. As we didn't increase the
refcount, this may lead to the entries being free'd while we iterate
over them in the first thread.

Fix the issue by properly handling the lifecycle of the backend's
entries via `config_file_entries_take` and `git_config_entries_free`,
respectively.

The function to take a reference to the config file's config entries
currently returns the reference via return value. Due to this, it's
harder than necessary to integrate into our typical coding style, as one
needs to make sure that a proper error code is set before erroring out
from the caller. This bites us in `config_file_delete`, where we call
`goto out` directly when `config_file_entries_take` returns `NULL`, but
we actually forget to set up the error code and thus return success.

Fix the issue by refactoring the function to return an error code and
pass the reference via an out-pointer.

As with the predecessing commit, this commit renames backend functions
of the configuration file backend. This helps to clearly separate
functionality and also to be able to see from backtraces which backend
is currently in use.

The configuration snapshot backend has been extracted from the old files
backend back in 2bff84ba (config_file: separate out read-only backend,
2019-07-26). To keep code churn manageable, the local functions weren't
renamed yet and thus still have references to the old diskfile backend.
Rename them accordingly to make them easier to understand.

Fix crash if snapshotting a config_snapshot

fix a bug introduced in 8a23597b

reflogs: fix behaviour around reflogs with newlines

Signed-off-by: Sven Strickroth <email@cs-ware.de>

commit: verify objects exist in git_commit_with_signature

There can be a significant difference between the system where we created the
buffer (if at all) and when the caller provides us with the contents of a
commit.

Verify that the commit we are being asked to create references objects which do
exist in the target repository.

There can be a significant difference between the system where we created the
buffer (if at all) and when the caller provides us with the contents of a
commit.

Provide some test cases (we have to adapt the existing ones because they refer
to trees and commits which do not exist).