git: add references for Git pull request heads When a pull request is created, stick a reference to it in the refs/pull namespace, similarly to what GitHub does, and remove it when the pull request is deleted. That reference guarantees the commits stay around and don't get garbage-collected when a PR is rebased or revised. Also, that makes it possible to access head of historic pull requests using Git by fetching a ref from the source repository. Unlike GitHub though, we don't put the ref into the destination repository and don't copy commits there to prevent undesired repository growth. Kallithea uses global pull request identifiers, so there should not be any confusion as to what pull request the ref corresponds to. We may later provide a shim to redirect users to the source repository if that deems needed.

[run]
omit =
    # the bin scripts are not part of the Kallithea web app
    kallithea/bin/*
    # we ship with no active extensions
    kallithea/config/rcextensions/*
    # dbmigrate and paster_commands are not part of the Kallithea web app
    kallithea/lib/dbmigrate/*
    kallithea/lib/paster_commands/*
    # the tests themselves should not be part of the coverage report
    kallithea/tests/*
    # the scm hooks are not run in the kallithea process
    kallithea/config/post_receive_tmpl.py
    kallithea/config/pre_receive_tmpl.py