get_urlmatch: avoid useless strbuf write

get_urlmatch: avoid useless strbuf write

We create a strbuf only to insert a single string, pass the
resulting buffer to a function (which does not modify the
string), and then free it. We can just pass the original
string instead.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>