Fix fetch --dry-run to not invoke git
Fix fetch --dry-run to not invoke git

This was broken by 209a681f9cb0e955d5659216f262ef5b00861e8a,
which replaced self.run() with git_common.run(), thus
bypassing self.run()'s dry_run check.

Bug: 788562
Change-Id: I9964c169b0ff56778c4660acab7ef7c010e37a36
Reviewed-on: https://chromium-review.googlesource.com/797611
Commit-Queue: Aaron Gable <bbed39beedae4cdb499af742d2fcd6c63934d93b@chromium.org>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools