Support for git archive stored tags

Support for git archive stored tags

Attempt to solve problem with git archive generated tarballs
(used for example by github when release is downloaded) which are no
longer git tree and (in contrast to officially released tarballs) also
doesn't contain .tarball-version file so git-version-gen script simply
cannot obtain valid version info.

Solution is based on using gitattributes which is instructs git to
replace string in the .gitarchivever file by known ref names.
git-version-gen is enhanced to support this file and tries to parse
any string which looks like "tag: v[0-9][^,)]*". If such string
is found it's used as a version. This file is used as a last attempt and
other methods (.tarball-version, git abbrev) have precedence.

Based on idea stated by Jan Pokorný <jpokorny@redhat.com>.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>