(cherry picked from commit f5c0535)

libfm: fix duplicate inclusion of libfm-extra

When there is no .version-suffix file in nixpkgs (like when fetching
nixpkgs with builtins.fetchGit), lib.version suffixes the version string
with "pre-git". The "pre" bit is special cased in
builtins.compareVersions which means "20.03pre-git" is interpreted as
"less than 20.03". This is clearly wrong for the release-20.03 branch
*after* the release has been made.

Change the suffix to "post-git" to make code like this behave the same
whether nixpkgs is fetched from git or the channel (which has
.version-suffix file):

  lib.versionOlder lib.version "20.03"
  lib.versionAtLeast lib.version "20.03"

(Currently the result depend on how nixpkgs was obtained!)

This change should be made part of the release process.

https://github.com/yory8/clipman/releases/tag/v1.5.2
(cherry picked from commit ebeea74)