Try to make "git gui" work again

based on #1912, some old upstreaming process: https://github.com/gitgitgadget/git/pull/612/

In theory the script needs some way to detect a MSYS2 mingw env and special case some
things on top of that in combination with is_Cygwin.

Since we want to keep this diff small, keep it hacky.

git/git-2.35.1-git-gui.patch

@@ -0,0 +1,25 @@
+--- git-2.35.1/git-gui/git-gui.sh.orig	2022-01-29 01:48:42.000000000 +0100
++++ git-2.35.1/git-gui/git-gui.sh	2022-03-12 18:51:13.610007600 +0100
+@@ -51,7 +51,10 @@
+ if { [info exists ::env(GIT_GUI_LIB_DIR) ] } {
+ 	set oguilib $::env(GIT_GUI_LIB_DIR)
+ } else {
+-	set oguilib {@@GITGUI_LIBDIR@@}
++	set oguilib [exec cygpath \
++			--windows \
++			--absolute \
++			{@@GITGUI_LIBDIR@@}]
+ }
+ set oguirel {@@GITGUI_RELATIVE@@}
+ if {$oguirel eq {1}} {
+@@ -274,10 +277,6 @@
+ 				set _iscygwin 0
+ 			} else {
+ 				set _iscygwin 1
+-				# Handle MSys2 which is only cygwin when MSYSTEM is MSYS.
+-				if {[info exists ::env(MSYSTEM)] && $::env(MSYSTEM) ne "MSYS"} {
+-					set _iscygwin 0
+-				}
+ 			}
+ 		} else {
+ 			set _iscygwin 0