[RFC PATCH 1/1] rebase --onto: Skip previously applied commits

[Cristian Ciocaltea <cristian.ciocaltea@collabora.com>]

When rebase is used with '--onto <newbase>', the patches that might have
been already applied on <newbase> are not detected, unless they resolve
to an empty commit. When the related files contain additional changes
merged, the rebase operation fails due to conflicts that require manual
intervention.

Ensure the '--onto' variant behaviour is consistent with the common
rebase by dropping the already applied commits on the target branch.

Note the current behavior is still reachable by using the
'--reapply-cherry-picks' flag.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
---
 builtin/rebase.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index b22768ca5b9f..2907c6db5cce 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1659,8 +1659,12 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		strbuf_addstr(&buf, "...");
 		strbuf_addstr(&buf, branch_name);
 		options.onto_name = xstrdup(buf.buf);
-	} else if (!options.onto_name)
+	} else if (!options.onto_name) {
 		options.onto_name = options.upstream_name;
+	} else if (options.upstream) {
+		options.restrict_revision = options.upstream;
+		options.upstream = NULL;
+	}
 	if (strstr(options.onto_name, "...")) {
 		if (get_oid_mb(options.onto_name, &branch_base) < 0) {
 			if (keep_base)
-- 
2.38.1