[pass] Using X primary selection and fix for git pull

[Matthieu Weber]

Hello, 

I started using password-store a few weeks ago, and I ran into a few minor
problems. You'll find next small patches fixing two of these.

The first is that I am used to pasting by middle-clicking the mouse.
Since the password is placed in the "clipboard" X selection, this is not
possible (you can paste into e.g., Firefox with ctrl-v, but not with
shift-insert or middle-click, and ctrl-v does not paste in Xterms). So I added
an environment variable to configure which one of "clipboard" or "primary"
must be used. "clipboard" remains the default value.

The second is that for some reason (my git version is too old?), "pass git pull"
fails with the following error: 
  fatal: /usr/lib/git-core/git-pull cannot be used without a working tree.
The solution is to "cd $GIT_WORK_TREE" before executing git, which should be
harmless. An alternative would be to unset GIT_WORK_TREE, but I fear it would
break some other git command :(

I hope you will find those patches useful.

Best regards,
-- 
Matthieu