Use python2 from environment

Some environments might have a old and/or broken version of python2
installed in /usr/bin but still wish to use git-imerge. The non-broken
version of python might be loaded separately from the base operating
system image via for example environment modules [1].

So use the currently active python2 from the environment instead of
hard coding the path to /usr/bin/python2.

[1] http://modules.sourceforge.net/