grep.c: extract show_line_header()

grep.c: extract show_line_header()

The grep code invokes show_line() to display the contents of a matched
or context line in its output. Part of this execution is to print a line
header that includes information such as the kind, the line- and
column-number and etc. of that match.

To prepare for the addition of an option to print only the matching
component(s) of a non-context line, we must prepare for the possibility
that a single line may contain multiple matching parts, and thus will
need multiple headers printed for a single line.

Extracting show_line_header allows us to do just that. In the subsequent
commit, it will be used within the colorization loop to print out only
the matching parts of a line, optionally with LFs delimiting
sub-matches.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>