hg-fast-export: work still if git-commands are not in PATH

hg-fast-export: work still if git-commands are not in PATH

In git 1.6.0, most git tools with a dash in the name will no
longer be installed in $bindir.  This patch makes hg-fast-export
use the "git <command>" form so it will work even if "git" is
the only piece of git machinery in the user's PATH.

On the other hand, the "git <command>" form does not help for
sourcing a shell script (with ".").  So use the full path to
source "git-sh-setup".

Signed-off-by: Jonathan Nieder <jrnieder@uchicago.edu>