Fix git.SetMTimes implementation
Fix git.SetMTimes implementation

Under my prodding, @SteveMaude refactored the code to get rid of
multiple git implementations.

Unfortunately, I missed the fact that the implementations were subtly
different. We accidentally took the one that was 10x slower. @djui
noticed this, which prompted me to find and fix the issue.

This has the effect of partially reverting
c804654739a55c0f31efdd4e177a26844881a7e2.

scraperwiki/hanoverd,sensiblecodeio/hanoverd,sensiblecodeio/hanoverd,scraperwiki/hanoverd