sequencer: extract revert message formatting into shared function

The logic for formatting revert commit messages (handling "Revert" and
"Reapply" cases) is currently duplicated between sequencer.c and will be
needed by builtin/replay.c.

Extract this logic into a new sequencer_format_revert_header() function
that can be shared. The function handles both regular reverts ("Revert
"<subject>"") and revert-of-revert cases ("Reapply "<subject>"").
When an oid is provided, the function appends the full commit hash and
period; otherwise the caller should append the commit reference.

Update do_pick_commit() to use the new helper, eliminating code
duplication while preserving the special handling for commit_use_reference.

Signed-off-by: Siddharth Asthana <siddharthasthana31@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: edith007

sequencer.c

@@ -2356,31 +2356,17 @@ static int do_pick_commit(struct repository *r,
 	 */
 
 	if (command == TODO_REVERT) {
-		const char *orig_subject;
-
 		base = commit;
 		base_label = msg.label;
 		next = parent;
 		next_label = msg.parent_label;
 		if (opts->commit_use_reference) {
 			strbuf_commented_addf(&ctx->message, comment_line_str,
 				"*** SAY WHY WE ARE REVERTING ON THE TITLE LINE ***");
-		} else if (skip_prefix(msg.subject, "Revert \"", &orig_subject) &&
-			   /*
-			    * We don't touch pre-existing repeated reverts, because
-			    * theoretically these can be nested arbitrarily deeply,
-			    * thus requiring excessive complexity to deal with.
-			    */
-			   !starts_with(orig_subject, "Revert \"")) {
-			strbuf_addstr(&ctx->message, "Reapply \"");
-			strbuf_addstr(&ctx->message, orig_subject);
-			strbuf_addstr(&ctx->message, "\n");
+			strbuf_addstr(&ctx->message, "\nThis reverts commit ");
 		} else {
-			strbuf_addstr(&ctx->message, "Revert \"");
-			strbuf_addstr(&ctx->message, msg.subject);
-			strbuf_addstr(&ctx->message, "\"\n");
+			sequencer_format_revert_header(&ctx->message, msg.subject, NULL);
 		}
-		strbuf_addstr(&ctx->message, "\nThis reverts commit ");
 		refer_to_commit(opts, &ctx->message, commit);
 
 		if (commit->parents && commit->parents->next) {
@@ -5572,6 +5558,35 @@ int sequencer_pick_revisions(struct repository *r,
 	return res;
 }
 
+void sequencer_format_revert_header(struct strbuf *out,
+				    const char *orig_subject,
+				    const struct object_id *oid)
+{
+	const char *revert_subject;
+
+	if (skip_prefix(orig_subject, "Revert \"", &revert_subject) &&
+	    /*
+	     * We don't touch pre-existing repeated reverts, because
+	     * theoretically these can be nested arbitrarily deeply,
+	     * thus requiring excessive complexity to deal with.
+	     */
+	    !starts_with(revert_subject, "Revert \"")) {
+		strbuf_addstr(out, "Reapply \"");
+		strbuf_addstr(out, revert_subject);
+		strbuf_addch(out, '\n');
+	} else {
+		strbuf_addstr(out, "Revert \"");
+		strbuf_addstr(out, orig_subject);
+		strbuf_addstr(out, "\"\n");
+	}
+
+	strbuf_addstr(out, "\nThis reverts commit ");
+	if (oid) {
+		strbuf_addstr(out, oid_to_hex(oid));
+		strbuf_addstr(out, ".\n");
+	}
+}
+
 void append_signoff(struct strbuf *msgbuf, size_t ignore_footer, unsigned flag)
 {
 	unsigned no_dup_sob = flag & APPEND_SIGNOFF_DEDUP;

sequencer.h

@@ -271,4 +271,15 @@ int sequencer_determine_whence(struct repository *r, enum commit_whence *whence)
  */
 int sequencer_get_update_refs_state(const char *wt_dir, struct string_list *refs);
 
+/*
+ * Formats a revert commit message following standard Git conventions.
+ * Handles both regular reverts ("Revert \"<subject>\"") and revert of revert
+ * cases ("Reapply \"<subject>\""). Adds "This reverts commit <oid>." if oid
+ * is provided, otherwise just adds "This reverts commit " and the caller
+ * should append the commit reference.
+ */
+void sequencer_format_revert_header(struct strbuf *out,
+				    const char *orig_subject,
+				    const struct object_id *oid);
+
 #endif /* SEQUENCER_H */