Recent changes in git have broken cg-log.  git-rev-list no longer
prints "commit" in front of commit hashes.  It turn out a local
"prefix" variable in main() shadows a file-scoped "prefix" variable.

The patch removed the local "prefix" variable since its value is never
used (in the intended way, that is).  The call to
setup_git_directory() is kept since it has useful side effects.

The file-scoped "prefix" variable is renamed to "commit_prefix" just
in case someone reintroduces "prefix" to hold the return value of
setup_git_directory().
Signed-off-by: NPavel Roskin <proski@gnu.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>