pip: pip install -e inconsistent checkout with git url and branch

Hi,

The way pip install -e works is inconsistent when you specify the branch name in a git url.

Look at the following behaviours :

Specifying a branch, example master on pip source repository:

$ pip install -e git+https://github.com/pypa/pip.git@master#egg=pip
$ cd src/pip/
$ git status
HEAD detached at 08ebc0a
nothing to commit, working directory clean

Specifying a branch again, develop from the same repo this time :

$ pip install -e git+https://github.com/pypa/pip.git@develop#egg=pip
$ cd src/pip/
$ git status
On branch develop
Your branch is up-to-date with 'origin/develop'.
nothing to commit, working directory clean

I would not expect to have 2 different states for the same command, depending on the branch I specified is the default one or not. Because I specified it in both cases, this should behave in the same way. Any further work with this editable installation becomes dependant on whether I am in the first case, or the second one.