rebase: initial support for multiple destinations This patch defines `SRC` (a single source revision) and `ALLSRC` (all source revisions) to be valid names in `--dest` revset if `--src` or `--rev` is used. So destination could be defined differently according to source revisions. The names are capitalized to make it clear they are "dynamically defined", distinguishable from normal revsets (Thanks Augie for the suggestion). This is useful, for example, `-r 'orphan()' -d 'calc-dest(SRC)'` to solve instability, which seems to be a highly wanted feature. The feature is not completed, namely if `-d` overlaps with `-r`, things could go wrong. A later patch will handle that case. The feature is also gated by `experimental.rebase.multidest` config option which is default off. Differential Revision: https://phab.mercurial-scm.org/D469