abuild: only change $pkgver from snapshot() if $reporev is not set
If the snapshot() functions creates a tarball for a specific repository revision (e.g. a specific tag) it doesn't make much sense to use the current date in the $pkgver as the tag may represent a release of the packaged software. This is useful in case upstream only provides a git repository with tags but not tarballs.
For example: aports!16095 (closed)