gitk: require git >= 2.20

gitk: require git >= 2.20

gitk has alternate code paths for early git up to 1.72, and has no
defined minimum version. Setting any version > 1.72 as minimum will
allow removing those code paths.

The recent set of advisories published for git, gitk, and git-gui add
updates for v2.43 and later, but Debian has buster withgit 2.20 still
available.  While Debian would be responsible for backporting any fixes
to such an early version, we have no good reason preclude it.
So, make 2.20 the minimum required git version.

Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>