gitk: Comply with XDG base directory specification

Write the gitk config data to $XDG_CONFIG_HOME/git/gitk ($HOME/.config/git/gitk
by default) in line with the XDG specification. This makes it consistent with
git which also follows the spec.

If $HOME/.gitk already exists use that for backward compatibility, so only new
installations are affected.
Signed-off-by: NAstril Hayato <astrilhayato@gmail.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -2787,14 +2787,17 @@ proc savestuff {w} {
     global linkfgcolor circleoutlinecolor
     global autoselect autosellen extdifftool perfile_attrs markbgcolor use_ttk
     global hideremotes want_ttk maxrefs
+    global config_file config_file_tmp
 
     if {$stuffsaved} return
     if {![winfo viewable .]} return
     catch {
-	if {[file exists ~/.gitk-new]} {file delete -force ~/.gitk-new}
-	set f [open "~/.gitk-new" w]
+	if {[file exists $config_file_tmp]} {
+	    file delete -force $config_file_tmp
+	}
+	set f [open $config_file_tmp w]
 	if {$::tcl_platform(platform) eq {windows}} {
-	    file attributes "~/.gitk-new" -hidden true
+	    file attributes $config_file_tmp -hidden true
 	}
 	puts $f [list set mainfont $mainfont]
 	puts $f [list set textfont $textfont]
@@ -2871,7 +2874,7 @@ proc savestuff {w} {
 	}
 	puts $f "}"
 	close $f
-	file rename -force "~/.gitk-new" "~/.gitk"
+	file rename -force $config_file_tmp $config_file
     }
     set stuffsaved 1
 }
@@ -12084,7 +12087,29 @@ namespace import ::msgcat::mc
 ## And eventually load the actual message catalog
 ::msgcat::mcload $gitk_msgsdir
 
-catch {source ~/.gitk}
+catch {
+    # follow the XDG base directory specification by default. See
+    # http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
+    if {[info exists env(XDG_CONFIG_HOME)] && $env(XDG_CONFIG_HOME) ne ""} {
+	# XDG_CONFIG_HOME environment variable is set
+	set config_file [file join $env(XDG_CONFIG_HOME) git gitk]
+	set config_file_tmp [file join $env(XDG_CONFIG_HOME) git gitk-tmp]
+    } else {
+	# default XDG_CONFIG_HOME
+	set config_file "~/.config/git/gitk"
+	set config_file_tmp "~/.config/git/gitk-tmp"
+    }
+    if {![file exists $config_file]} {
+	# for backward compatibility use the old config file if it exists
+	if {[file exists "~/.gitk"]} {
+	    set config_file "~/.gitk"
+	    set config_file_tmp "~/.gitk-tmp"
+	} elseif {![file exists [file dirname $config_file]]} {
+	    file mkdir [file dirname $config_file]
+	}
+    }
+    source $config_file
+}
 
 parsefont mainfont $mainfont
 eval font create mainfont [fontflags mainfont]