Verify all the .git/ paths will be handled by the migration logic before
starting the migration. This way we still abort & log an error, but the
user gets to see it before we put the tree into a state that they have to
manually recover. Also add a few more known-safe-to-clobber paths.
Bug: https://crbug.com/gerrit/15273
Change-Id: If49d69b341bc960ddcafa30da333fb5ec7145b51
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/327557
Reviewed-by: Colin Cross <ccross@android.com>
Tested-by: Mike Frysinger <vapier@google.com>