grep: stop modifying buffer in show_line()

grep: stop modifying buffer in show_line()

When showing lines via grep (or looking for funcnames), we call
show_line() on a multi-line buffer. It finds the end of line and marks
it with a NUL. However, we don't need to do so, as the resulting line is
only used along with its "eol" marker:

 - we pass both to next_match(), which takes care to look at only the
   bytes we specified

 - we pass the line to output_color() without its matching eol marker.
   However, we do use the "match" struct we got from next_match() to
   tell it how many bytes to look at (which can never exceed the string
   we passed it).

So we can stop setting and restoring this NUL marker. That makes the
code simpler, and will allow us to take a const buffer in a future
patch.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>