Add explicit path of git installation by 'git --exec-path'.

As pointed out by documentation, the correct use of 'git-sh-setup' is
using $(git --exec-path) to avoid problems with not standard
installations.

Signed-off-by: gianluca.pacchiella <pacchiel@studenti.ph.unito.it>

git-subtree.sh

@@ -27,7 +27,7 @@ rejoin        merge the new branch back into HEAD
 squash        merge subtree changes as a single commit
 "
 eval $(echo "$OPTS_SPEC" | git rev-parse --parseopt -- "$@" || echo exit $?)
-. git-sh-setup
+. $(git --exec-path)/git-sh-setup
 require_work_tree
 
 quiet=