workingctx: eliminate remove function
Inlining it into it's last remaining call place in cmdutil.copy.
Note that cmdutil.copy is called with the wlock already held, so no additional
locking is needed to call util.unlinkpath.
We do not need to wrap the util.unlinkpath call into a try block, because
at that point we already know whether abssrc exists or not -- thanks to the
preceding util.copyfile call. Adding a new local 'srcexists' in cmdutil.copy
for that purpose.
color: check if ui is already a subclass of colorui before wrapping it
since it's possible to reuse the ui object (see
80c599eee3f3)
between dispatch calls, the ui might have been wrapped already.
we could probably move this test up a bit but leaving it here
allows updates to take effect by extstyles() and configstyles().