gitk: Fix GIT_TRACE issues

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: Aske Olsson <askeolsson@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>