fix display of page name in recentchanges after a revert

fix display of page name in recentchanges after a revert

When the wiki is in a subdir of the git repo, a web revert would show
in recentchanges as eg, doc/index, instead of just index.

This happened because decode_git_file caches a $prefix that is dependant
on the $git_dir setting, and the revert code runs with a different
$git_dir, which polluted the $prefix for later.

Fix this by adding a with_git_dir that juggles the variables properly.