git-new-workdir: Fix shell warning about operator == used with test.

Use = instead of == with test to test for equality.
Signed-off-by: NSimon Hausmann <simon@lst.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/workdir/git-new-workdir

@@ -24,7 +24,7 @@ git_dir=$(cd "$orig_git" 2>/dev/null &&
   git rev-parse --git-dir 2>/dev/null) ||
   die "\"$orig_git\" is not a git repository!"
 
-if test "$git_dir" == ".git"
+if test "$git_dir" = ".git"
 then
 	git_dir="$orig_git/.git"
 fi