Consume Git with fix for multi-pack-index verify

[jrbriggs]

Git multi-pack-index verify can produce a false negative when run on an enlistment with a large number of packs. This issue tracks consuming the fix in Git to not exhaust file descriptors.

[jeffhostetler]

See microsoft/git#121

[jeffhostetler]

TODO upstream this.

[jeffhostetler]

See also microsoft/git#124