mergetool-lib: Don't use deprecated variable to detect 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 replace this using XDG_CURRENT_DESKTOP instead, and cleanup the code to avoid duplication and supporting KDE's kdiff3 better.

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

Marco Trevisan (Treviño) (2):
mergetool-lib: use $XDG_CURRENT_DESKTOP to check GNOME
mergetool-lib: give kdiff3 priority in KDE environments

 git-mergetool--lib.sh | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

base-commit: 85b4e0a6dc8407de6f69808d9ee6debdf167ced3
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-693%2F3v1n0%2Fdesktop-envs-fixes-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-693/3v1n0/desktop-envs-fixes-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/693

Range-diff vs v1:

1: 66a026ae67 ! 1: 26b25b0d65 mergetool-lib: use $XDG_CURRENT_DESKTOP to check GNOME

     @@ Commit message
          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 check the XDG_CURRENT_DESKTOP env variable, that
     -    is supported by all the desktop environments.
     +    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 go through all the values to ensure
     -    we're using GNOME.
     +    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
      
     @@ git-mergetool--lib.sh: run_merge_cmd () {
       }
       
      +is_desktop () {
     -+	IFS=':'
     -+	for desktop in ${XDG_CURRENT_DESKTOP}
     -+	do
     -+		if test "$desktop" = "$1"
     -+		then
     -+			return 0
     -+		fi
     -+	done
     -+
     -+	return 1
     ++	case ":$XDG_CURRENT_DESKTOP:" in
     ++	*:$1:*)
     ++		return 0
     ++		;;
     ++	*)
     ++		return 1
     ++		;;
     ++	esac
      +}
      +
       list_merge_tool_candidates () {
     @@ git-mergetool--lib.sh: list_merge_tool_candidates () {
       	if test -n "$DISPLAY"
       	then
      -		if test -n "$GNOME_DESKTOP_SESSION_ID"
     -+		if is_desktop "GNOME"
     ++		if is_desktop "GNOME" || test -n "$GNOME_DESKTOP_SESSION_ID"
       		then
       			tools="meld opendiff kdiff3 tkdiff xxdiff $tools"
       		else
 2:  fc0d2b103e < -:  ---------- mergetool-lib: keep a list of cross desktop merge tools
 3:  37090d2322 ! 2:  c18a5edf50 mergetool-lib: give kdiff3 prioirty in KDE environments

@@ Metadata
Author: Marco Trevisan (Treviño) <[email protected]>

## Commit message ##
- mergetool-lib: give kdiff3 prioirty in KDE environments
+ mergetool-lib: give kdiff3 priority in KDE environments

## git-mergetool--lib.sh ##
@@ git-mergetool--lib.sh: list_merge_tool_candidates () {
- fi
- if test -n "$DISPLAY"
- then
-- cross_desktop_tools="opendiff kdiff3 tkdiff xxdiff"
-+ cross_desktop_tools="opendiff tkdiff xxdiff"
- if is_desktop "GNOME"
+ if is_desktop "GNOME" || test -n "$GNOME_DESKTOP_SESSION_ID"
then
-- tools="meld $cross_desktop_tools $tools"
-+ tools="meld $cross_desktop_tools kdiff3 $tools"
-+ elif is_desktop "KDE"
+ tools="meld opendiff kdiff3 tkdiff xxdiff $tools"
++ elif is_desktop "KDE" || test x"$KDE_FULL_SESSION" = x"true"
+ then
-+ tools="kdiff3 $cross_desktop_tools meld $tools"
++ tools="kdiff3 opendiff tkdiff xxdiff meld $tools"
else
-- tools="$cross_desktop_tools meld $tools"
-+ tools="$cross_desktop_tools kdiff3 meld $tools"
+ tools="opendiff kdiff3 tkdiff xxdiff meld $tools"
fi
- tools="$tools gvimdiff diffuse diffmerge ecmerge"
- tools="$tools p4merge araxis bc codecompare"

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