revset: add function to build dict of positional and keyword arguments
Keyword arguments will be convenient for functions that will take more than
one optional or boolean flags. For example,
file(pattern[, subrepos=false])
subrepo([[pattern], status])
Because I don't think all functions should accept key=value syntax, getkwargs()
does not support variadic functions such as 'ancestor(*changeset)'.
The core logic is placed in the parser module because keyword arguments will
be more useful in the templater, where functions take more options. Test cases
will be added by the next patch.