[PATCH 10/11] builtin rebase: only store fully-qualified refs in `options.head_name`

[Pratik Karki <predatoramigo@gmail.com>]

When running a rebase on a detached HEAD, we currently store the string
"detached HEAD" in options.head_name. That is a faithful translation of
the shell script version, and we still kind of need it for the purposes of
the scripted backends.

It is poor style for C, though, where we would really only want a valid,
fully-qualified ref name as value, and NULL for detached HEADs, using
"detached HEAD" for display only. Make it so.

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
---
 builtin/rebase.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index a261f552f1..63634210c0 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -169,7 +169,8 @@ static int run_specific_rebase(struct rebase_options *opts)
 	add_var(&script_snippet, "upstream_name", opts->upstream_name);
 	add_var(&script_snippet, "upstream",
 				 oid_to_hex(&opts->upstream->object.oid));
-	add_var(&script_snippet, "head_name", opts->head_name);
+	add_var(&script_snippet, "head_name",
+		opts->head_name ? opts->head_name : "detached HEAD");
 	add_var(&script_snippet, "orig_head", oid_to_hex(&opts->orig_head));
 	add_var(&script_snippet, "onto", oid_to_hex(&opts->onto->object.oid));
 	add_var(&script_snippet, "onto_name", opts->onto_name);
@@ -251,6 +252,9 @@ static int reset_head(struct object_id *oid, const char *action,
 		*old_orig = NULL, oid_old_orig;
 	int ret = 0;
 
+	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)
 		return -1;
 
@@ -558,7 +562,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	 * branch_name -- branch/commit being rebased, or
 	 * 		  HEAD (already detached)
 	 * orig_head -- commit object name of tip of the branch before rebasing
-	 * head_name -- refs/heads/<that-branch> or "detached HEAD"
+	 * head_name -- refs/heads/<that-branch> or NULL (detached HEAD)
 	 */
 	if (argc > 0)
 		 die("TODO: handle switch_to");
@@ -575,7 +579,8 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 				branch_name = options.head_name;
 
 		} else {
-			options.head_name = xstrdup("detached HEAD");
+			free(options.head_name);
+			options.head_name = NULL;
 			branch_name = "HEAD";
 		}
 		if (get_oid("HEAD", &options.orig_head))
-- 
2.18.0