add dashdash to basename call for safety

It is possible to have an argument to ``basename`` in git-sh-setup.sh that begins with a dash, causing an error. This commit adds a -- so that the argument won't break the call by being misinterpreted.

In the following example, the argument passed to ``basename`` will be "-bash":

    $ (source $(git --exec-path)/git-sh-setup require_work_tree)
    basename: illegal option -- b
    usage: basename string [suffix]
           basename [-a] [-s suffix] string [...] 

Environment:
OSX Yosemite
GNU bash, version 3.2.53(1)-release (x86_64-apple-darwin14)
git version 2.1.2