ident: trim trailing newline from /etc/mailname

We use fgets to read the /etc/mailname file, which means we
will typically end up with an extra newline in our
git_default_email. Most of the time this doesn't matter, as
fmt_ident will skip it as cruft, but there is one code path
that accesses it directly (in http-push.c:lock_remote).

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

ident.c

@@ -74,6 +74,10 @@ static int add_mailname_host(char *buf, size_t len)
 	}
 	/* success! */
 	fclose(mailname);
+
+	len = strlen(buf);
+	if (len && buf[len-1] == '\n')
+		buf[len-1] = '\0';
 	return 0;
 }