allow --help to run when called outside a repository

Don't try to run Git commands before having an opportunity to run
'usage' and other help commands.

Fixes #

git-latexdiff

@@ -198,7 +198,6 @@ bibtex=0
 biber=0
 output=
 initial_dir=$PWD
-initial_repo=$(git rev-parse --show-toplevel)
 tmpdir_prefix="/tmp"
 prepare_cmd=
 flatten=1
@@ -377,6 +376,8 @@ while test $# -ne 0; do
     shift
 done
 
+initial_repo=$(git rev-parse --show-toplevel)
+
 if test -z "$new" ; then
     new=HEAD
 fi