Easy fixes for 'git push' performance

[derrickstolee]

This PR does two things that are super-easy fixes for improving 'git push' performance.

1. Set pack.useBitmaps to false on mount time.
2. Update Git to Git-2.19.1.gvfs.1.3.ge56ee8b, which includes a change to stop checking ambiguous refs during 'git pack-objects'. This reduces possibly thousands of file reads of the form:

.git/<oid>
.git/refs/<oid>
.git/refs/tags/<oid>
.git/refs/heads/<oid>
.git/refs/remotes/<oid>
.git/refs/remotes/<oid>/HEAD

By setting the config value, we don't perform these file checks for the object ids that are passed into stdin.

For (1), this PR sets pack.useBitmaps to false for VFS for Git enlistments. This config value defaults to true, and during git pack-objects we check for the existence of a .bitmap file for every packfile, even those covered by the multi-pack-index.

Each of these checks took 2-3 seconds on my small test. I hope to get better numbers before merging, but want to see our scale tests.