Speed up git destination with mostly excluded files

[aaron-skydio]

When migrating into a git destination with excluded files, copybara calls git add on all of these excluded files individually. For cases where the destination repo is large, but most of the files are excluded, this drastically slows down the import process and is by far the slowest part of the import, taking minutes when the rest of the import takes seconds.

This change looks for directories where all files are excluded (i.e. we plan to call git add with every file in a directory), and instead calls git add with just the directory. It also does this recursively, i.e. on entire directory trees of this form. With this change, this part of the code is no longer the bottleneck in this type of migration.

There is potentially some overlap here with #112 - however, if I understand correctly, that PR would only work for CHANGE_REQUEST workflows, while this implementation will speed up all workflows with git destinations. And, I don't see any reason why these two changes are mutually exclusive.

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[mikelalcon]

We had to rollback the change in e2a64da due to some bug. We are investigating.