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.

Sets up a linux host to prepare for a build.

Refactor citest.sh to enable local testing by developers.

Add citest.ps1 PowerShell script to run the tests.

Simplify the names for the tests, removing the unnecessary
"libgit2-clar" prefix.  Make "all" the new default test run, and include
the online tests by default (since HTTPS should always be enabled).

For the CI tests, create an offline-only test, then the various online
tests.

mbedtls: remove unused variable "cacert"

In commit 382ed1e8 (mbedtls: load default CA certificates, 2018-03-29),
the function `git_mbedtls_stream_global_init` was refactored to call out
to `git_mbedtls__set_cert_location` instead of setting up the
certificates itself. The conversion forgot to remove the now-unused
"cacert" variable, which is now only getting declared to be free'd at
the end of the function. Remove it.