ident: do not drop username when reading from /etc/mailname

An earlier conversion from fgets() to strbuf_getline() in the
codepath to read from /etc/mailname to learn the default host-part
of the ident e-mail address forgot that strbuf_getline() stores the
line at the beginning of the buffer just like fgets().

The "username@" the caller has prepared in the strbuf, expecting the
function to append the host-part to it, was lost because of this.
Reported-by: NMihai Rusu <dizzy@google.com>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Acked-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

ident.c

@@ -41,6 +41,7 @@ static void copy_gecos(const struct passwd *w, struct strbuf *name)
 static int add_mailname_host(struct strbuf *buf)
 {
 	FILE *mailname;
+	struct strbuf mailnamebuf = STRBUF_INIT;
 
 	mailname = fopen("/etc/mailname", "r");
 	if (!mailname) {
@@ -49,14 +50,17 @@ static int add_mailname_host(struct strbuf *buf)
 				strerror(errno));
 		return -1;
 	}
-	if (strbuf_getline(buf, mailname, '\n') == EOF) {
+	if (strbuf_getline(&mailnamebuf, mailname, '\n') == EOF) {
 		if (ferror(mailname))
 			warning("cannot read /etc/mailname: %s",
 				strerror(errno));
+		strbuf_release(&mailnamebuf);
 		fclose(mailname);
 		return -1;
 	}
 	/* success! */
+	strbuf_addbuf(buf, &mailnamebuf);
+	strbuf_release(&mailnamebuf);
 	fclose(mailname);
 	return 0;
 }