gitk: Handle external diff tool with spaces in the path

gitk: Handle external diff tool with spaces in the path

This fixes the launching of external diff to handle a diff tool
that has spaces in the path.  This ensures a correctly formed
tcl list is passed to the open command with a single pipe character
prefixing the list (as per the tcl manual page for open).

The specific fault observed was that selecting WinMerge as the diff
tool from the default installed location in Program Files failed to
be launched from the context menu.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>