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 <redacted>
Acked-by: Linus Torvalds <redacted>
Signed-off-by: Junio C Hamano <redacted>