filetime_from_git plugin: add timezone information to datetime objects

Time-zone information was lacking in datetimes for filetime_from_git plugin, and present elsewhere in Pelican, leading to bad comparisons (e.g. "CRITICAL: can't compare offset-naive and offset-aware datetimes"). Time zone information is now added to datetime objects returned by this plugin by using a utility function in Pelican.

This bug is similar to that in #264