magit.el: execute git grep on toplevel directory

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
magit.el: use word-at-point as default input to grep

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Merge commit '25b5f6e3ca7f9892ce607121aaf5c4409b994b19' into artagnon/grep
fix byte-compilation
Merge branch 'artagnon/grep'
Merge branch 'master' into next