(Build-)depend on git >= 2.22 to avoid memory leaked git being bundled or used

(Build-)depend on git >= 2.22 to avoid memory leaked git being bundled or used

Note from Joey:
  git-annex still supports git 2.1, but operates in a degraded fashion.
  It would be better for backports of the debian package to also
  backport a newer git. This dependency is mostly expressing that,
  also that any users who might upgrade git-annex should also upgrade
  git.

  Also worth noting that the i386ancient autobuilder has git 2.1 on it
  (best I have been able to manage there), but luckily the epoch is
  bumped to 2, so the dependencies will still be satisfied.