gitk: Skip translation of "wrong Tcl version" message

gitk: Skip translation of "wrong Tcl version" message

We check the required Tcl version number before we setup msgcat for
language translation.  If the Tcl version is too old just display the
untranslated error text.

The caller of show_error can now pass an alternative function for mc.
The Tcl list function turns the translation into a no-op.

This fixes the error:
    Error in startup script: invalid command name "mc"
when attempting to start gitk with Tcl 8.3.

Tested with both Tcl 8.3 and 8.4.

Signed-off-by: Bernt Hansen <bernt@norang.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>