am: simplify allocations in get_commit_info()

After we call split_ident_line(), we have several begin/end
pairs for various parts of the ident. We then copy each into
a strbuf to create a single string, and then detach that
string.  We can instead skip the strbuf entirely and just
duplicate the strings directly.

This is shorter, and it makes it more obvious that we are
not leaking the strbuf (we were not before, because every
code path either died or hit a strbuf_detach).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

builtin/am.c

@@ -1376,40 +1376,35 @@ static int get_mail_commit_oid(struct object_id *commit_id, const char *mail)
  */
 static void get_commit_info(struct am_state *state, struct commit *commit)
 {
-	const char *buffer, *ident_line, *author_date, *msg;
+	const char *buffer, *ident_line, *msg;
 	size_t ident_len;
 	struct ident_split ident_split;
-	struct strbuf sb = STRBUF_INIT;
 
 	buffer = logmsg_reencode(commit, NULL, get_commit_output_encoding());
 
 	ident_line = find_commit_header(buffer, "author", &ident_len);
 
-	if (split_ident_line(&ident_split, ident_line, ident_len) < 0) {
-		strbuf_add(&sb, ident_line, ident_len);
-		die(_("invalid ident line: %s"), sb.buf);
-	}
+	if (split_ident_line(&ident_split, ident_line, ident_len) < 0)
+		die(_("invalid ident line: %.*s"), (int)ident_len, ident_line);
 
 	assert(!state->author_name);
 	if (ident_split.name_begin) {
-		strbuf_add(&sb, ident_split.name_begin,
-			ident_split.name_end - ident_split.name_begin);
-		state->author_name = strbuf_detach(&sb, NULL);
+		state->author_name =
+			xmemdupz(ident_split.name_begin,
+				 ident_split.name_end - ident_split.name_begin);
 	} else
 		state->author_name = xstrdup("");
 
 	assert(!state->author_email);
 	if (ident_split.mail_begin) {
-		strbuf_add(&sb, ident_split.mail_begin,
-			ident_split.mail_end - ident_split.mail_begin);
-		state->author_email = strbuf_detach(&sb, NULL);
+		state->author_email =
+			xmemdupz(ident_split.mail_begin,
+				 ident_split.mail_end - ident_split.mail_begin);
 	} else
 		state->author_email = xstrdup("");
 
-	author_date = show_ident_date(&ident_split, DATE_MODE(NORMAL));
-	strbuf_addstr(&sb, author_date);
 	assert(!state->author_date);
-	state->author_date = strbuf_detach(&sb, NULL);
+	state->author_date = xstrdup(show_ident_date(&ident_split, DATE_MODE(NORMAL)));
 
 	assert(!state->msg);
 	msg = strstr(buffer, "\n\n");