fakeupstream.cgi: implement upstream=github_releases

related bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1026886

Allow upstream=github_releases to avoid unexpected failure to do packaging pre-release as an official release.

Here is the failure scenario:

1. upstream make a tag as v1.0.0
2. upstream releases v1.0.0 as pre-release
3. upstream remove a tag v1.0.0
4. upstream make a tag v1.0.0 again
5. upstream releases v1.0.0 as official release

If you overlooked step 2 as a pre-release, d/watch just notify v1.0.0 released and shows it in package overview [1]

As a result, it may cause wrong version packaged into archive. If so, you need to apply many patch sets against to v1.0.0-2.

[1] https://qa.debian.org/developer.php?login=[your account]

These kind of failure is derived from difficulty of ditinguish pre-release from https://github.com/org/project/releases or https://github.com/org/project/tags.

Usage in debian/watch (assumption):

version=4
opts="searchmode=plain" \
  http://qa.local/cgi-bin/fakeupstream.cgi?upstream=github_releases/google/sentencepiece \
  /sentencepiece-@ANY_VERSION@@ARCHIVE_EXT@