completion: support git range-diff

Tab completion of `git range-diff` is very convenient, especially
given that the revision arguments to specify the commit ranges to
compare are typically more complex than, say, what is normally passed
to `git log`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1976,6 +1976,20 @@ _git_push ()
 	__git_complete_remote_or_refspec
 }
 
+_git_range_diff ()
+{
+	case "$cur" in
+	--*)
+		__gitcomp "
+			--creation-factor= --dual-color
+			$__git_diff_common_options
+		"
+		return
+		;;
+	esac
+	__git_complete_revlist
+}
+
 _git_rebase ()
 {
 	__git_find_repo_path