Improve extdiff configuration.
In addition to the old cmd.foo, opts.foo hgrc entries, allow a more simple
alias = command [opts]... form. For example:
[extdiff]
cdiff = colordiff -uprN
adding a
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding b
pushing to ../a
abort: could not lock repository ../a: Permission denied