bash shorthands like `~` don't work with --git-dir argument

[asmeurer]

I was playing around with http://nbviewer.ipython.org/urls/raw.github.com/tarmstrong/code-analysis/master/IPythonReviewTime.ipynb and noticed two issues:

• The --git-dir expects the .git dir, but the error messages suggest it is looking for one directory up. See Better error messages when invalid git-dir specified #5.
• It does not support paths with ~