...rpmbuild otherwise expands valid macro names in changelog entries.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

The "Simple versioning" scheme:
  https://docs.fedoraproject.org/en-US/packaging-guidelines/Versioning/#_simple_versioning

probably doesn't apply to passt, given that upstream git tags are
not really releases. Switch to the "Snapshots" versioning scheme:
  https://docs.fedoraproject.org/en-US/packaging-guidelines/Versioning/#_snapshots

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

This is required as Fedora doesn't accept a temporary pointer to
a source URL.

Reported-by: Ralf Corsepius <rc040203@freenet.de>
Reported-by: Artur Frenszek-Iwicki <fedora@svgames.pl>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

git_dir_changelog is useful in theory, but it requires pairs of
annotated tags, which should be generated by rpkg itself to make any
sense, implying a relatively heavyweight interaction whenever I want
to push a new package version.

Also, the default content of the changelog entries include the full
list of changes, but the Fedora Packaging Guidelines specifically
mention that:

  [t]hey must never simply contain an entire copy of the source
  CHANGELOG entries.

We don't have a CHANGELOG file, but the full git history is
conceptually equivalent for this purpose, I guess.

Introduce our own passt_git_changelog() rpkg macro, building
changelog entries, using tags in the form DATE-SHA, where DATE
is an ISO 8601 date representation, and SHA is a short (7-digits)
form of the head commit at a given moment (git push).

These changelog entries mention, specifically, changes to the
packaging information itself (entries under contrib/fedora), and
simply report a link to cgit for the ranges between tags.

Reported-by: Benson Muite <benson_muite@emailplus.org>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

...they seem to be supported by COPR now and make things simpler.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>