Added support for BSD style systems.

The stat command on BSD style systems, doesn't have the -c switch used in utils/git.sh, so made a feature detection on the stat command, to determine which switch to use.

I also had a problem with the $[$()-$()] syntax that was used in the to determine time_diff of the modifed time on the FETCH_HEAD. So I fixed that as well.

Using FreeBSD 9.1.
Tested the changes doesn't break homeshick on a Linux system (Debian squeeze)