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>

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();
+	}
 }

t/t5520-pull.sh

@@ -237,6 +237,23 @@ test_expect_success '--rebase' '
 	test new = "$(git show HEAD:file2)"
 '
 
+test_expect_success '--rebase fast forward' '
+	git reset --hard before-rebase &&
+	git checkout -b ff &&
+	echo another modification >file &&
+	git commit -m third file &&
+
+	git checkout to-rebase &&
+	git pull --rebase . ff &&
+	test "$(git rev-parse HEAD)" = "$(git rev-parse ff)" &&
+
+	# The above only validates the result.  Did we actually bypass rebase?
+	git reflog -1 >reflog.actual &&
+	sed "s/^[0-9a-f][0-9a-f]*/OBJID/" reflog.actual >reflog.fuzzy &&
+	echo "OBJID HEAD@{0}: pull --rebase . ff: Fast-forward" >reflog.expected &&
+	test_cmp reflog.expected reflog.fuzzy
+'
+
 test_expect_success '--rebase fails with multiple branches' '
 	git reset --hard before-rebase &&
 	test_must_fail git pull --rebase . copy master 2>err &&