sequencer: export and rename subject_length()

This function can be used in other parts of git. Let's move the
function to commit.c and also rename it to make the name of the
function more generic.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Charvi Mendiratta <charvi077@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -535,6 +535,20 @@ int find_commit_subject(const char *commit_buffer, const char **subject)
 	return eol - p;
 }
 
+size_t commit_subject_length(const char *body)
+{
+	const char *p = body;
+	while (*p) {
+		const char *next = skip_blank_lines(p);
+		if (next != p)
+			break;
+		p = strchrnul(p, '\n');
+		if (*p)
+			p++;
+	}
+	return p - body;
+}
+
 struct commit_list *commit_list_insert(struct commit *item, struct commit_list **list_p)
 {
 	struct commit_list *new_list = xmalloc(sizeof(struct commit_list));

commit.h

@@ -165,6 +165,9 @@ const void *detach_commit_buffer(struct commit *, unsigned long *sizep);
 /* Find beginning and length of commit subject. */
 int find_commit_subject(const char *commit_buffer, const char **subject);
 
+/* Return length of the commit subject from commit log message. */
+size_t commit_subject_length(const char *body);
+
 struct commit_list *commit_list_insert(struct commit *item,
 					struct commit_list **list);
 int commit_list_contains(struct commit *item,

sequencer.c

@@ -1724,20 +1724,6 @@ enum todo_item_flags {
 	TODO_EDIT_FIXUP_MSG    = (1 << 2),
 };
 
-static size_t subject_length(const char *body)
-{
-	const char *p = body;
-	while (*p) {
-		const char *next = skip_blank_lines(p);
-		if (next != p)
-			break;
-		p = strchrnul(p, '\n');
-		if (*p)
-			p++;
-	}
-	return p - body;
-}
-
 static const char first_commit_msg_str[] = N_("This is the 1st commit message:");
 static const char nth_commit_msg_fmt[] = N_("This is the commit message #%d:");
 static const char skip_first_commit_msg_str[] = N_("The 1st commit message will be skipped:");
@@ -1861,7 +1847,7 @@ static int append_squash_message(struct strbuf *buf, const char *body,
 	if (starts_with(body, "amend!") ||
 	    ((command == TODO_SQUASH || seen_squash(opts)) &&
 	     (starts_with(body, "squash!") || starts_with(body, "fixup!"))))
-		commented_len = subject_length(body);
+		commented_len = commit_subject_length(body);
 
 	strbuf_addf(buf, "\n%c ", comment_line_char);
 	strbuf_addf(buf, _(nth_commit_msg_fmt),