Fixes issue #1072. Currently, `git-lfs` installs filters which misinterpret
filenames with leading dashes as command line flags. Separate paths with a
double dash (`--`) when installing filters so these files are handled
properly.
The failing test now passes:
```
$ ./test/test-unusual-filenames.sh
...
test: push unusually named files ... OK
test: pull unusually named files ... OK
```
