This saves a network hop when using git and passing an explicit sha
as a ref by comparing the version that's already checked out.
Yields a ~4x speedup on my local machine
Before:
```
$ /usr/local/bin/pip --version
pip 7.1.0 from /usr/local/lib/python2.7/site-packages (python 2.7)
$ time /usr/local/bin/pip install --disable-pip-version-check -e git+https://github.com/getsentry/raven-python.git@56fc6f7beecf445843d0ec7052bb8c6f0ea80a2e#egg=raven_dev
Obtaining raven-dev from git+https://github.com/getsentry/raven-python.git@56fc6f7beecf445843d0ec7052bb8c6f0ea80a2e#egg=raven_dev
Updating ./src/raven-dev clone (to 56fc6f7beecf445843d0ec7052bb8c6f0ea80a2e)
Could not find a tag or branch '56fc6f7beecf445843d0ec7052bb8c6f0ea80a2e', assuming commit.
Installing collected packages: raven-dev
Running setup.py develop for raven-dev
Successfully installed raven-dev
/usr/local/bin/pip install --disable-pip-version-check -e 0.84s user 0.48s system 39% cpu 3.300 total
```
After:
```
$ /Users/matt/.virtualenvs/pip/bin/pip --version
pip 7.2.0.dev0 from /Users/matt/code/pip (python 2.7)
$ time /Users/matt/.virtualenvs/pip/bin/pip install --disable-pip-version-check -e git+https://github.com/getsentry/raven-python.git@56fc6f7beecf445843d0ec7052bb8c6f0ea80a2e#egg=raven_dev
Obtaining raven-dev from git+https://github.com/getsentry/raven-python.git@56fc6f7beecf445843d0ec7052bb8c6f0ea80a2e#egg=raven_dev
checking version
Skipping because already up-to-date.
Installing collected packages: raven-dev
Running setup.py develop for raven-dev
Successfully installed raven-dev
/Users/matt/.virtualenvs/pip/bin/pip install --disable-pip-version-check -e 0.59s user 0.22s system 98% cpu 0.824 total
```
This catches the BadCommand that get raised by vcs modules when the VCS
software is not installed or not in the PATH. In this case, we behave as
if the .git, .svn or .hg folder hadn't been found at all (but with the
correct warning).
