Add a function to update HEAD after creating a commit

Add update_head_with_reflog() based on the code that updates HEAD
after committing in builtin/commit.c that can be called by 'git
commit' and other commands.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -1591,13 +1591,11 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	struct strbuf sb = STRBUF_INIT;
 	struct strbuf author_ident = STRBUF_INIT;
 	const char *index_file, *reflog_msg;
-	char *nl;
 	struct object_id oid;
 	struct commit_list *parents = NULL;
 	struct stat statbuf;
 	struct commit *current_head = NULL;
 	struct commit_extra_header *extra = NULL;
-	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
 
 	if (argc == 2 && !strcmp(argv[1], "-h"))
@@ -1720,25 +1718,11 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	strbuf_release(&author_ident);
 	free_commit_extra_headers(extra);
 
-	nl = strchr(sb.buf, '\n');
-	if (nl)
-		strbuf_setlen(&sb, nl + 1 - sb.buf);
-	else
-		strbuf_addch(&sb, '\n');
-	strbuf_insert(&sb, 0, reflog_msg, strlen(reflog_msg));
-	strbuf_insert(&sb, strlen(reflog_msg), ": ", 2);
-
-	transaction = ref_transaction_begin(&err);
-	if (!transaction ||
-	    ref_transaction_update(transaction, "HEAD", &oid,
-				   current_head
-				   ? &current_head->object.oid : &null_oid,
-				   0, sb.buf, &err) ||
-	    ref_transaction_commit(transaction, &err)) {
+	if (update_head_with_reflog(current_head, &oid, reflog_msg, &sb,
+				    &err)) {
 		rollback_index_files();
 		die("%s", err.buf);
 	}
-	ref_transaction_free(transaction);
 
 	unlink(git_path_cherry_pick_head());
 	unlink(git_path_revert_head());

sequencer.c

@@ -1,10 +1,10 @@
 #include "cache.h"
 #include "config.h"
 #include "lockfile.h"
-#include "sequencer.h"
 #include "dir.h"
 #include "object.h"
 #include "commit.h"
+#include "sequencer.h"
 #include "tag.h"
 #include "run-command.h"
 #include "exec_cmd.h"
@@ -752,6 +752,43 @@ int template_untouched(const struct strbuf *sb, const char *template_file,
 	return rest_is_empty(sb, start - sb->buf);
 }
 
+int update_head_with_reflog(const struct commit *old_head,
+			    const struct object_id *new_head,
+			    const char *action, const struct strbuf *msg,
+			    struct strbuf *err)
+{
+	struct ref_transaction *transaction;
+	struct strbuf sb = STRBUF_INIT;
+	const char *nl;
+	int ret = 0;
+
+	if (action) {
+		strbuf_addstr(&sb, action);
+		strbuf_addstr(&sb, ": ");
+	}
+
+	nl = strchr(msg->buf, '\n');
+	if (nl) {
+		strbuf_add(&sb, msg->buf, nl + 1 - msg->buf);
+	} else {
+		strbuf_addbuf(&sb, msg);
+		strbuf_addch(&sb, '\n');
+	}
+
+	transaction = ref_transaction_begin(err);
+	if (!transaction ||
+	    ref_transaction_update(transaction, "HEAD", new_head,
+				   old_head ? &old_head->object.oid : &null_oid,
+				   0, sb.buf, err) ||
+	    ref_transaction_commit(transaction, err)) {
+		ret = -1;
+	}
+	ref_transaction_free(transaction);
+	strbuf_release(&sb);
+
+	return ret;
+}
+
 static int is_original_commit_empty(struct commit *commit)
 {
 	const struct object_id *ptree_oid;

sequencer.h

@@ -69,4 +69,8 @@ int message_is_empty(const struct strbuf *sb,
 		     enum commit_msg_cleanup_mode cleanup_mode);
 int template_untouched(const struct strbuf *sb, const char *template_file,
 		       enum commit_msg_cleanup_mode cleanup_mode);
+int update_head_with_reflog(const struct commit *old_head,
+			    const struct object_id *new_head,
+			    const char *action, const struct strbuf *msg,
+			    struct strbuf *err);
 #endif