gitk: Fix GIT_TRACE issues

Check if GIT_TRACE env var is set and unset it if it is.

If the environment var GIT_TRACE=1 exists gitk will fail when trying
to find gitdir:
$ git rev-parse --git-dir
trace: built-in: git 'rev-parse' '--git-dir'
.git

Other git commands will also show debug output hence not work as
intended.
Signed-off-by: NAske Olsson <askeolsson@gmail.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -11532,6 +11532,11 @@ if {[catch {package require Tk 8.4} err]} {
     exit 1
 }
 
+# Unset GIT_TRACE var if set
+if { [info exists ::env(GIT_TRACE)] } {
+    unset ::env(GIT_TRACE)
+}
+
 # defaults...
 set wrcomcmd "git diff-tree --stdin -p --pretty"