contrib/hgfixes/__init__.py

revset: use '%' as an operator for 'only'

With this patch, we can make it much easier to specify 'only(A,B)' ->
A%B. Similarly, 'only(A)' -> A%.

On Windows, '%' is a semi-reserved symbol in the following way: using non-bash
shells (e.g. cmd.exe but NOT PowerShell, ConEmu, and cmder), %var% is only
expanded when 'var' exists and is surrounded by '%'.

That only leaves batch scripts which could prove to be problematic. I posit
that this isn't a big issue because any developer of batch scripts already
knows that to use '%' one needs to escape it by using a double '%%'.

Alternatives to '%' could be '=' but that might be limiting our future if we
ever decide to use temporary assignments in a revset.