Fix #267, git-missing did not catch branch name.

It was a bug on my setup, the test did no detect the
branch name, so I change:
if [[ $arg != -* ]]; then
by
if [ -n "${arg}" ]; then

According to test man page, -n:
«True if the length of string is non-zero.»

I also added quote and {} around the param,
to prevent bash expension around strange char.
Merge pull request #268 from GuillaumeSeren/bug/git-missing

Fix #267, git-missing did not catch branch name.