t7810: add missing variables to tests in loop

t7810: add missing variables to tests in loop

Some tests in t7810-grep.sh are in a loop that runs them against HEAD and
the work tree.  In order for that to work the test code should use the
variables $L (display name), $H (HEAD or empty string) and $HC (revision
prefix for result lines); otherwise tests are just repeated with the same
target.  Add the variables where they're missing and make sure the test
description is wrapped in double quotes (instead of single quotes) to
allow variables to be expanded.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>