ci: add optional test variables

ci: add optional test variables

The commit-graph and multi-pack-index features introduce optional
data structures that are not required for normal Git operations.
It is important to run the normal test suite without them enabled,
but it is helpful to also run the test suite using them.

Our continuous integration scripts include a second test stage that
runs with optional GIT_TEST_* variables enabled. Add the following
two variables to that stage:

  GIT_TEST_COMMIT_GRAPH
  GIT_TEST_MULTI_PACK_INDEX

This will slow down the operation, as we build a commit-graph file
after every 'git commit' operation and build a multi-pack-index
during every 'git repack' operation. However, it is important that
future changes are compatible with these features.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>