Fix R1:R2 syntax for git revision-range.

The refactoring for the revisions/files project caused a regression in the
0.5.3 release when using --revision-range with git. This change fixes it to
split on ':'. This change will be for the release-0.5.x branch only, since the
master branch uses the new command-line syntax.

Reviewed at https://reviews.reviewboard.org/r/5209/

Fixes bug 3182.