Fix _forgit_diff on macOS (#375)

In bash 3.2 uninitialized arrays contain one entry: an empty string,
whereas in modern versions of bash they do not contain any entry. Make
sure the $commits and $files arrays are explicitly initialized as empty
arrays to allow git to fall back to diffing local changes instead of
trying to diff against a revision with a name of an empty string.

bin/git-forgit

@@ -242,6 +242,8 @@ _forgit_diff_enter() {
 _forgit_diff() {
     _forgit_inside_work_tree || return 1
     local files opts commits escaped_commits
+    commits=()
+    files=()
     [[ $# -ne 0 ]] && {
         if git rev-parse "$1" -- &>/dev/null ; then
             if [[ $# -gt 1 ]] && git rev-parse "$2" -- &>/dev/null; then