grep: Add the option '--open-files-in-pager'

grep: Add the option '--open-files-in-pager'

This adds an option to open the matching files in the pager, and if the
pager happens to be "less" (or "vi") and there is only one grep pattern,
it also jumps to the first match right away.

So, 'git grep -P abc' is a short form for 'less +/abc $(grep -l abc)'
except that it works also with spaces in file names, and it does not
start the pager if there was no matching file.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>