Use grep -F when no argument is passed (literal string match)

Why?
Most users who pass the current selection to grep likely do not intend to pass
the selection as a regex input string.

This makes the grepcmd use an additional -F flag to perform literal-string
matching for the current selection. The -F flag seems to be the standard flag
for literal-string matching in every grep implementation I've found.

0 files changed, 0 insertions, 0 deletions