Remove `--short` git option

`pull-tools` script fails on Ubuntu 12.04 because it's git does not
support `--short` option in `git symbolic-ref` command.
To fix it, `--short` option was removed and `cut` command to shorten the
name was added.

Change-Id: Ia7be559b7edd0d78da40ed49e88fd191c21ec13c
Merge "Remove `--short` git option"