mergetool-lib: use $XDG_CURRENT_DESKTOP to check GNOME

To list merge tool candidates we used to use a private GNOME env
variable (GNOME_DESKTOP_SESSION_ID) that has been deprecated for long time ago
and removed as part of GNOME 3.30.0 release [1].

So, git should instead primarily check the XDG_CURRENT_DESKTOP env variable,
that is now supported by all the desktop environments.

Since the variable is actually a colon-separated list of names that the current
desktop is known as, we need to check if the value is set if we're using GNOME.

[1] https://gitlab.gnome.org/GNOME/gnome-session/-/commit/00e0e6226371d53f65

Signed-off-by: Marco Trevisan (Treviño) <mail@3v1n0.net>

Author: 3v1n0

git-mergetool--lib.sh

@@ -266,6 +266,17 @@ run_merge_cmd () {
 	fi
 }
 
+is_desktop () {
+	case ":$XDG_CURRENT_DESKTOP:" in
+	*:$1:*)
+		return 0
+		;;
+	*)
+		return 1
+		;;
+	esac
+}
+
 list_merge_tool_candidates () {
 	if merge_mode
 	then
@@ -275,7 +286,7 @@ list_merge_tool_candidates () {
 	fi
 	if test -n "$DISPLAY"
 	then
-		if test -n "$GNOME_DESKTOP_SESSION_ID"
+		if is_desktop "GNOME" || if test -n "$GNOME_DESKTOP_SESSION_ID"
 		then
 			tools="meld opendiff kdiff3 tkdiff xxdiff $tools"
 		else