pull: fast-forward "pull --rebase=true"

"git pull --rebase" always runs "git rebase" after fetching the
commit to serve as the new base, even when the new base is a
descendant of the current HEAD, i.e. we haven't done any work.

In such a case, we can instead fast-forward to the new base without
invoking the rebase process.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 18 insertions, 4 deletions

diff --git a/builtin/pull.c b/builtin/pull.c
index bf3fd3f..2a41d41 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -878,10 +878,24 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 		if (merge_heads.nr > 1)
 			die(_("Cannot merge multiple branches into empty head."));
 		return pull_into_void(*merge_heads.sha1, curr_head);
-	} else if (opt_rebase) {
-		if (merge_heads.nr > 1)
-			die(_("Cannot rebase onto multiple branches."));
+	}
+	if (opt_rebase && merge_heads.nr > 1)
+		die(_("Cannot rebase onto multiple branches."));
+
+	if (opt_rebase) {
+		struct commit_list *list = NULL;
+		struct commit *merge_head, *head;
+
+		head = lookup_commit_reference(orig_head);
+		commit_list_insert(head, &list);
+		merge_head = lookup_commit_reference(merge_heads.sha1[0]);
+		if (is_descendant_of(merge_head, list)) {
+			/* we can fast-forward this without invoking rebase */
+			opt_ff = "--ff-only";
+			return run_merge();
+		}
 		return run_rebase(curr_head, *merge_heads.sha1, rebase_fork_point);
-	} else
+	} else {
 		return run_merge();
+	}
 }