Introduce `git_apply`, which will take a `git_diff` and apply it to the
working directory (akin to `git apply`), the index (akin to `git apply
--cached`), or both (akin to `git apply --index`).

The generic `git_reader` interface simplifies `git_apply_tree` somewhat.
Reimplement `git_apply_tree` with them.

Similar to the `git_iterator` interface, the `git_reader` interface will
allow us to read file contents from an arbitrary repository-backed data
source (trees, index, or working directory).

Introduce `git_apply_tree`, which will apply a `git_diff` to a given
`git_tree`, allowing an in-memory patch application for a repository.

Optionally hash the contents of files encountered in the filesystem or
working directory iterators.  This is not expected to be used in
production code paths, but may allow us to simplify some test contexts.

For working directory iterators, apply filters as appropriate, since we
have the context able to do it.

CI: Fix macOS leak detection

README: more CI status badges

ci: Fix some minor issues

Don't prefix the path to the yaml templates - the nightly template
itself is already in the `azure-pipelines` directory.  Instead, just
use the relative path.

POSIX: the CMakeLists.txt configures the test names; when we query
ctest for the test command-line to run, fail if the tests are not
found.

Win32: The CMakeLists.txt configures the test names; when we query
ctest for the test command-line to run, fail if the tests are not
found.

Object parse fixes

Windows CI: fail build on test failure

ci: run all the jobs during nightly builds

Instead of running the oddball builds, run all the builds (the ones
that we always run during PR validation and CI) during a nightly
build for increased coverage.

PowerShell can _read_ top-level variables in functions, but cannot _update_
top-level variables in functions unless they're explicitly prefixed with
`$global`.

The commit message encoding is currently being parsed by the
`git__prefixcmp` function. As this function does not accept a buffer
length, it will happily skip over a buffer's end if it is not `NUL`
terminated.

Fix the issue by using `git__prefixncmp` instead. Add a test that
verifies that we are unable to parse the encoding field if it's cut off
by the supplied buffer length.

We currently do not have any test suites dedicated to parsing commits
from their raw representations. Add one based on `git_object__from_raw`
to be able to test special cases more easily.

When parsing tags, we skip all unknown fields that appear before the tag
message. This skipping is done by using a plain `strstr(buffer, "\n\n")`
to search for the two newlines that separate tag fields from tag
message. As it is not possible to supply a buffer length to `strstr`,
this call may skip over the buffer's end and thus result in an out of
bounds read. As `strstr` may return a pointer that is out of bounds, the
following computation of `buffer_end - buffer` will overflow and result
in an allocation of an invalid length.

Fix the issue by using `git__memmem` instead. Add a test that verifies
parsing the tag fails not due to the allocation failure but due to the
tag having no message.

While the tests in object::tag::read exercises reading and parsing valid
tags from the ODB, they barely try to verify that the parser fails in a
sane way when parsing invalid tags. Create a new test suite
object::tag::parse that directly exercise the parser by using
`git_object__from_raw` and add various tests for valid and invalid tags.

Unfortunately, neither the `memmem` nor the `strnstr` functions are part
of any C standard but are merely extensions of C that are implemented by
e.g. glibc. Thus, there is no standardized way to search for a string in
a block of memory with a limited size, and using `strstr` is to be
considered unsafe in case where the buffer has not been sanitized. In
fact, there are some uses of `strstr` in exactly that unsafe way in our
codebase.

Provide a new function `git__memmem` that implements the `memmem`
semantics. That is in a given haystack of `n` bytes, search for the
occurrence of a byte sequence of `m` bytes and return a pointer to the
first occurrence. The implementation chosen is the "Not So Naive"
algorithm from [1]. It was chosen as the implementation is comparably
simple while still being reasonably efficient in most cases.
Preprocessing happens in constant time and space, searching has a time
complexity of O(n*m) with a slightly sub-linear average case.

[1]: http://www-igm.univ-mlv.fr/~lecroq/string/

strtol removal

 buf::oom tests: use custom allocator for oom failures

Create a custom allocator for the `buf::oom` tests that will fail with
out-of-memory errors in predictable ways.  We were previously trying to
guess the way that various allocators on various platforms would fail
in a way such that `malloc`/`realloc` would return `NULL` (instead of
aborting the application, or appearing suspicious to various
instrumentation or static code analysis tools like valgrind.)

Introduce a fake `malloc` and `realloc` that will return `NULL` on
allocations requesting more than 100 bytes.  Otherwise, we proxy to the
default allocator.  (It's important to use the _default_ allocator, not
just call `malloc`, since the default allocator on Windows CI builds may
be the debugging C runtime allocators which would not be compatible with
a standard `malloc`.)

Provide a utility to reset custom allocators back to their default.
This is particularly useful for testing.

ci: arm docker builds

We don't need two separate docker images for OpenSSL and mbedTLS.
They've been combined into a single image `trusty-amd64` that supports
both.

On a 32-bit Linux systems, the value large enough to make malloc
guarantee a failure is also large enough that valgrind considers it
"fishy".  Skip this test on those systems entirely.

Newer dependencies means newer places to leak!

Use Bionic so that we have a modern libssh2 (for communicating with
GitHub).  We've ported fixes to our Trusty-based amd64 images, but
maintaining patches for multiple platforms is heinous.

Bind the proxy specifically to 127.0.0.1 instead of all addresses.  This
is not strictly necessary for operations, but having a potentially open
proxy on a network is not a good idea.

Use multiarch arm32 and arm64 docker images to run Xenial-based images
for those platforms.  We can support all the tests on ARM32 and 64
_except_ the proxy-based tests.  Our proxy on ARM seems regrettably
unstable, either due to some shoddy dependencies (with native code?)
or the JREs themselves.

Run these platforms as part of our nightly builds; do not run them
during pull request or CI validation.

As the number of each grow, separate the CI build scripts from
the YAML definitions.

Win32 path canonicalization refactoring

Check object existence when creating a tree from an index