rebase: teach reset_head() to optionally skip the worktree

This is what the legacy (scripted) rebase does in
`move_to_original_branch`, and we will need this functionality in the
next commit.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

builtin/rebase.c

@@ -368,13 +368,15 @@ static void add_var(struct strbuf *buf, const char *name, const char *value)
 
 #define RESET_HEAD_DETACH (1<<0)
 #define RESET_HEAD_HARD (1<<1)
+#define RESET_HEAD_REFS_ONLY (1<<2)
 
 static int reset_head(struct object_id *oid, const char *action,
 		      const char *switch_to_branch, unsigned flags,
 		      const char *reflog_orig_head, const char *reflog_head)
 {
 	unsigned detach_head = flags & RESET_HEAD_DETACH;
 	unsigned reset_hard = flags & RESET_HEAD_HARD;
+	unsigned refs_only = flags & RESET_HEAD_REFS_ONLY;
 	struct object_id head_oid;
 	struct tree_desc desc[2] = { { NULL }, { NULL } };
 	struct lock_file lock = LOCK_INIT;
@@ -390,7 +392,7 @@ static int reset_head(struct object_id *oid, const char *action,
 	if (switch_to_branch && !starts_with(switch_to_branch, "refs/"))
 		BUG("Not a fully qualified branch: '%s'", switch_to_branch);
 
-	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0) {
+	if (!refs_only && hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0) {
 		ret = -1;
 		goto leave_reset_head;
 	}
@@ -403,6 +405,9 @@ static int reset_head(struct object_id *oid, const char *action,
 	if (!oid)
 		oid = &head_oid;
 
+	if (flags & RESET_HEAD_REFS_ONLY)
+		goto reset_head_refs;
+
 	memset(&unpack_tree_opts, 0, sizeof(unpack_tree_opts));
 	setup_unpack_trees_porcelain(&unpack_tree_opts, action);
 	unpack_tree_opts.head_idx = 1;
@@ -443,6 +448,7 @@ static int reset_head(struct object_id *oid, const char *action,
 		goto leave_reset_head;
 	}
 
+reset_head_refs:
 	reflog_action = getenv(GIT_REFLOG_ACTION_ENVIRONMENT);
 	strbuf_addf(&msg, "%s: ", reflog_action ? reflog_action : "rebase");
 	prefix_len = msg.len;