Add date to hg git commits

I needed to preserve dates so I added a few lines - thought I'd PR just in case this is useful and desired.

I don't understand how standard the hg patch format of having the date on two lines is - the times I looked at looked like this:

    # HG changeset patch
    # User Thomas Ballinger <thomasballinger@gmail.com>
    # Date 1400296253 14400
    #      Fri May 16 23:10:53 2014 -0400
    # Node ID 8ce6b7f45d798a53e88548f402ef3b2e91bce599
    # Parent  d549edd48fbe88ad3fc09d49a88bcf754c79963b
    tests for line accessors
    
    diff --git a/bpython/curtsiesfrontend/line.py b/bpython/curtsiesfrontend/line.py
    ...

I don't know if they always look like this, but this code assumes they do.