grep: handle pre context lines on demand

grep: handle pre context lines on demand

Factor out pre context line handling into the new function
show_pre_context() and change the algorithm to rewind by looking for
newline characters and roll forward again, instead of maintaining an
array of line beginnings and ends.

This is slower for hits, but the cost for non-matching lines becomes
zero.  Normally, there are far more non-matching lines, so the time
spent in total decreases.

Before this patch (current Linux kernel repo, best of five runs):

	$ time git grep --no-ext-grep -B1 memset >/dev/null

	real	0m2.134s
	user	0m1.932s
	sys	0m0.196s

	$ time git grep --no-ext-grep -B1000 memset >/dev/null

	real	0m12.059s
	user	0m11.837s
	sys	0m0.224s

The same with this patch:

	$ time git grep --no-ext-grep -B1 memset >/dev/null

	real	0m2.117s
	user	0m1.892s
	sys	0m0.228s

	$ time git grep --no-ext-grep -B1000 memset >/dev/null

	real	0m2.986s
	user	0m2.696s
	sys	0m0.288s

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>