grep: use REG_STARTEND (if available) to speed up regexec

grep: use REG_STARTEND (if available) to speed up regexec

BSD and glibc have an extension to regexec which takes a buffer + length pair
instead of a NUL-terminated string. Since we already have the length computed
this can save us a strlen call inside regexec.

Signed-off-by: Benjamin Kramer <benny.kra@googlemail.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>