Improve installer

Don't choke on non-directory or non-git directories: continue on and
print a warning.

This is useful so you can run `overcommit *` in a directory and just
install to all of the matching targets that are Git repositories.

Change-Id: Ie5b617f7753a617ac6405464aa3901b49b7fffd9
Reviewed-on: https://gerrit.causes.com/22813
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>