testing/git-absorb: link against system-provided libgit2 and move to community

requested review from @maribu

assigned to @jirutka

Thx!

added 4 commits

• 39483b07...b3c66324 - 2 commits from branch alpine:master
• ccb90539 - testing/git-absorb: link against system-provided libgit2
• ca1c02f7 - community/git-absorb: move from testing

Compare with previous version

merged