pull requests: fix same git repository pull-request issue Currentry, Kallithea can not create pull-request from same git repository. Because, ancestor variable is not initialized when create pull-request from same git repository. This patch find ancestor by `git merge-base` command and initialize it.

syntax: glob
*.pyc
*.swp
*.sqlite
*.tox
*.egg-info
*.egg

syntax: regexp
^rcextensions
^build
^dist/
^docs/build/
^docs/_build/
^data$
^sql_dumps/
^\.settings$
^\.project$
^\.pydevproject$
^\.coverage$
^kallithea\.db$
^test\.db$
^Kallithea\.egg-info$
^rc.*\.ini$
^fabfile.py
^\.idea$