postgresql: Update to latest versions

(cherry picked from commit a986bbf4ba569e4e88fe2cb34728d0b691495dd5)
release.nix: Drop officialRelease flag

(cherry picked from commit 3b616e378a81129ad69a56322506d6f45e6358b3)
Mark builds from git explicitly

(cherry picked from commit 0b091e1286c8a6f449b81f572573b08f1a3e06a5)
Shorten the version string

As suggested by Bjørn Forsman, use the number of commits in the
release branch.