contrib/completion: --line-number to git grep

The "-n" option of "git grep" gained a synonym "--line-number" with
commit 7d6cb10b ("grep: Add the option '--line-number'", 2011-03-28).

Teach bash-completion about it.

Signed-off-by: Michał Kiedrowicz <michal.kiedrowicz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1485,7 +1485,7 @@ _git_grep ()
 		__gitcomp "
 			--cached
 			--text --ignore-case --word-regexp --invert-match
-			--full-name
+			--full-name --line-number
 			--extended-regexp --basic-regexp --fixed-strings
 			--files-with-matches --name-only
 			--files-without-match