From: Phillip Wood <redacted>

Separate out calculating the merge base between 'onto' and 'HEAD' from
the check for whether we can fast-forward or not. This means we can skip
the fast-forward checks when the rebase is forced and avoid calculating
the merge-base between 'HEAD' and 'onto' when --keep-base is given.

Signed-off-by: Phillip Wood <redacted>
---
 builtin/rebase.c | 34 ++++++++++++++++++++--------------
 1 file changed, 20 insertions(+), 14 deletions(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index cbafcc41e75..40619a0fb2d 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c

@@ -871,13 +871,9 @@ static int can_fast_forward(struct commit *onto, struct commit *upstream,
 	struct commit_list *merge_bases = NULL;
 	int res = 0;
 
-	merge_bases = get_merge_bases(onto, head);
-	if (!merge_bases || merge_bases->next) {
-		oidcpy(branch_base, null_oid());
-		goto done;
-	}
+	if (is_null_oid(branch_base))
+		goto done; /* fill_branch_base() found multiple merge bases */
 
-	oidcpy(branch_base, &merge_bases->item->object.oid);
 	if (!oideq(branch_base, &onto->object.oid))
 		goto done;
 

@@ -887,7 +883,6 @@ static int can_fast_forward(struct commit *onto, struct commit *upstream,
 	if (!upstream)
 		goto done;
 
-	free_commit_list(merge_bases);
 	merge_bases = get_merge_bases(upstream, head);
 	if (!merge_bases || merge_bases->next)
 		goto done;

@@ -902,6 +897,20 @@ done:
 	return res && is_linear_history(onto, head);
 }
 
+static void fill_branch_base(struct rebase_options *options,
+			    struct object_id *branch_base)
+{
+	struct commit_list *merge_bases = NULL;
+
+	merge_bases = get_merge_bases(options->onto, options->orig_head);
+	if (!merge_bases || merge_bases->next)
+		oidcpy(branch_base, null_oid());
+	else
+		oidcpy(branch_base, &merge_bases->item->object.oid);
+
+	free_commit_list(merge_bases);
+}

@@ -1669,8 +1678,8 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		if (!options.onto)
 			die(_("Does not point to a valid commit '%s'"),
 				options.onto_name);
+		fill_branch_base(&options, &branch_base);
 	}
-
 	if (options.fork_point > 0)
 		options.restrict_revision =
 			get_fork_point(options.upstream_name, options.orig_head);