Released tar.gz should include correct stgit/builtin_version.py

[Necoro]

The current setup.py tries to read the version it is installing from the git repository. This, naturally, does not work when installing from the tarball. As a consequence, stgit reports 'unkown-version'.

Fix: Add the correct builtin_version.py to the tarball and remove the call to write_builtin_version from the released setup.py.

[jpgrayson]

This is repaired in v0.19.

The release archives (tar.gz and zip) now contain a .git_archival.txt file. This file is a template that is formatted by git/GitHub to contain the refs associated with the archive when the archive is created.

When running setup.py from an unpacked archive, .git_archival.txt will be parsed for tags. The first matching tag will be used for the version.

N.B. this mechanism will only work for archives coming from tags. For untagged archives, setup.py will fail to determine the version. This is because the information in .git_archival.txt is insufficient to derive the same kind of inter-tag version information available from git describe such as the last tag or the number of commits since the last tag.