fetch: convert strncmp() with strlen() to starts_with()

fetch: convert strncmp() with strlen() to starts_with()

Using strncmp() and strlen() to check whether a string starts with
another one requires repeating the prefix candidate.  Use starts_with()
instead, which reduces repetition and is more readable.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>