gitk: fix branch name encoding error

After "git checkout -b '漢字'" to create a branch with UTF-8
character in it, "gitk" shows the branch name incorrectly, as it
forgets to turn the bytes read from the "git show-ref" command
into Unicode characters.

Signed-off-by: Kazuhiro Kato <[email protected]>
Signed-off-by: Paul Mackerras <[email protected]>

Author: kkato233

gitk

@@ -1780,11 +1780,15 @@ proc readrefs {} {
     global otherrefids idotherrefs mainhead mainheadid
     global selecthead selectheadid
     global hideremotes
+    global tclencoding
 
     foreach v {tagids idtags headids idheads otherrefids idotherrefs} {
 	unset -nocomplain $v
     }
     set refd [open [list | git show-ref -d] r]
+    if {$tclencoding != {}} {
+	fconfigure $refd -encoding $tclencoding
+    }
     while {[gets $refd line] >= 0} {
 	if {[string index $line 40] ne " "} continue
 	set id [string range $line 0 39]