pull: avoid running both merge and rebase

When opt_rebase is true, we still first check if we can fast-forward.
If the branch is fast-forwardable, then we can avoid the rebase and just
use merge to do the fast-forward logic.  However, when commit a6d7eb2
("pull: optionally rebase submodules (remote submodule changes only)",
2017-06-23) added the ability to rebase submodules it accidentally
caused us to run BOTH a merge and a rebase.  Add a flag to avoid doing
both.

This was found when a user had both pull.rebase and rebase.autosquash
set to true.  In such a case, the running of both merge and rebase would
cause ORIG_HEAD to be updated twice (and match HEAD at the end instead
of the commit before the rebase started), against expectation.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pull.c

@@ -986,6 +986,7 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 
 	if (opt_rebase) {
 		int ret = 0;
+		int ran_ff = 0;
 		if ((recurse_submodules == RECURSE_SUBMODULES_ON ||
 		     recurse_submodules == RECURSE_SUBMODULES_ON_DEMAND) &&
 		    submodule_touches_in_range(the_repository, &rebase_fork_point, &curr_head))
@@ -1002,10 +1003,12 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 			if (is_descendant_of(merge_head, list)) {
 				/* we can fast-forward this without invoking rebase */
 				opt_ff = "--ff-only";
+				ran_ff = 1;
 				ret = run_merge();
 			}
 		}
-		ret = run_rebase(&curr_head, merge_heads.oid, &rebase_fork_point);
+		if (!ran_ff)
+			ret = run_rebase(&curr_head, merge_heads.oid, &rebase_fork_point);
 
 		if (!ret && (recurse_submodules == RECURSE_SUBMODULES_ON ||
 			     recurse_submodules == RECURSE_SUBMODULES_ON_DEMAND))