fetch: create status table using strbuf

fetch: create status table using strbuf

When we fetch from a remote, we print a status table like:

  From url
   * [new branch]   foo -> origin/foo

We create this table in a static buffer using sprintf. If
the remote refnames are long, they can overflow this buffer
and smash the stack.

Instead, let's use a strbuf to build the string.

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