Refactoring: create command for gittessera.
No title required for creating a Tessera.

When invoking
    git tessera create
no title is required, a tessea is created with the default
title.
correct path joining.
Refactoring: exported git interface abstraction.
Merge branch 'master' of github.com:neolynx/git-tessera

Conflicts:
	tessera/gitcommands.py
	tessera/gittessera.py