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.

Squash some leaks

Instead of allocating the ciphers_list, make it a static array.  This
prevents us from leaking it or having to manage its memory.

Don't just free the spec vector, also free the specs themselves.

Don't just free the push status structure, actually free the strings that were
strdup'd into the struct as well.

Free the url field when resetting the stream to avoid leaking it.