The packfile_raw fuzzer is using some internal APIs from libgit2, which
makes it hard to compile it as part of the oss-fuzz project. As oss-fuzz
requires us to link against the C++ FuzzingEngine library, we cannot use
"-DBUILD_FUZZERS=ON" directly but instead have to first compile an
object from our fuzzers and then link against the C++ library. Compiling
the fuzzer objects thus requires an external invocation of CC, and we
certainly don't want to do further black magic by adding libgit2's
private source directory to the header include path.

To fix the issue, convert the code to not use any internal APIs. Besides
some headers which we have to add now, this also requires us to change
to the hashing function of the ODB. Note that this will change the
hashing result, as we have previously not prepended the object header to
the data that is to be hashed. But this shouldn't matter in practice, as
we don't care for the hash value anyway.

Both the USE_SANITIZER and USE_COVERAGE options are convenience options
that turn on a set of CFLAGS. Despite our own set of CFLAGS required to
build libgit2, we have no real business to mess with them, though, as
they can easily be passed in by the user via specifying the CFLAGS
environment variable. The reasoning behind not providing them is that as
soon as we start adding those for some usecases, users might ask for
other sets of CFLAGS catering to their specific need in another usecase.
Thus, we do not want to support them here.

Right now, users are being instrucded to add the
"-DCMAKE_EXE_LINKER_FLAGS=-fsanitize=fuzzer" flag when they want to
build our fuzzers. This is error-prone and user unfriendly. Instead,
just add the flag to our fuzzers' build instructions so that it happens
automatically. Adjust the README accordingly.

We do want to notify users compiling our source code early on if they
try to use C flags which aren't supported. Add a new macro `AddCFlag`,
which results in a fatal error in case the flag is not supported, and
use it for our fuzzing flags.

Like all our other internal code, we want to force the use of C90 for
our fuzzers. Do so by setting the "C_STANDARD" property of our fuzzing
targets.

Our layout uses names like "examples" or "tests" which is why the "fuzz"
directory doesn't really fit in here. Rename the directory to be called
"fuzzers" instead. Furthermore, we rename the fuzzer "fuzz_packfile_raw"
to "packfile_raw_fuzzer", which is also in line with the already
existing fuzzer at google/oss-fuzz.

While at it, rename the "packfile_raw" fuzzer to instead just be called
"packfile" fuzzer.

This change adds support for building a fuzz target for exercising the
packfile parser, as well as documentation. It also runs the fuzzers in
Travis to avoid regressions.

ci: run VSTS builds on master and maint branches

Windows: default credentials / fallback credential handling

ci: add VSTS build badge to README

ci: set PKG_CONFIG_PATH for travis

Homebrew's formula for openssl is "keg-only", which means it does not
install it into /usr/local.  On macOS builds, we need to set
PKG_CONFIG_PATH to include it.

CI: Refactor and introduce VSTS builds

revwalk: remove tautologic condition for hiding a commit

Instead of trying to run coverity builds during the regular PR process,
run them during a regularly scheduled cron process.  These only need to
run nightly, so it makes sense to bring them out of the PR process.

The contition cannot be reached with `commit->uninteresting` being true:
either a `break` or a `continue` statement will be hit in this case.

winhttp: retry erroneously failing requests

The leaks process is not good about handling children.  Ensure that its
child is `nohup`ed so that the grandparent shell won't wait for it to
exit.

On Linux (where we run valgrind) allocate a smaller buffer, but still an
insanely large size.  This will cause malloc to fail but will not cause
valgrind to report a likely error with a negative-sized malloc.

Keep the original buffer size on non-Linux platforms: this is
well-tested on them and changing it may be problematic.  On macOS, for
example, using the new size causes `malloc` to print a warning to
stderr.

Add a configurable limit to the max pack size that will be indexed

Script to set up dependencies on a macOS build system.