Run fetch on git / hg packages

[mgsloan]

Currently, if you change the SHA of a downloaded git repo, it will try to do a git reset (hg update). If this doesn't work, then we do a full git clone. It would be more efficient to do a git fetch followed by git reset.

[snoyberg]

The new architecture of Pantry—which no longer keeps the repos on the filesystem—wouldn't allow for this. We could add back storage of the repos, but IMO that's overall a pessimization. Closing.