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>

gitk

@@ -9,6 +9,22 @@ exec wish "$0" -- "$@"
 
 package require Tk
 
+set MIN_GIT_VERSION 2.20
+regexp {^git version ([\d.]*\d)} [exec git version] _ git_version
+if {[package vcompare $git_version $MIN_GIT_VERSION] < 0} {
+    set message "The git executable found is too old.
+The minimum required version is $MIN_GIT_VERSION.0.
+The version of git found is $git_version."
+
+    catch {wm withdraw .}
+    tk_messageBox \
+        -icon error \
+        -type ok \
+        -title "gitk: fatal error" \
+        -message $message
+    exit 1
+}
+
 ######################################################################
 ##
 ## Enabling platform-specific code paths
@@ -12820,8 +12836,6 @@ if {$use_ttk} {
     setttkstyle
 }
 
-regexp {^git version ([\d.]*\d)} [exec git version] _ git_version
-
 set show_notes {}
 if {[package vcompare $git_version "1.6.6.2"] >= 0} {
     set show_notes "--show-notes"