rebase: sort destmap topologically Previously rebase source and destination could not overlap. But with the multi-destination support, source and destination could reasonably partially overlap. That requires another topological sort on `{sourcerev: destrev}` graph (destmap). This patch implements that. If a revision's destination is itself, the error message gets changed from "source is ancestor of destination" to "source and destination form a cycle". Not marking as BC since automation should depend on exit code, not error message. Differential Revision: https://phab.mercurial-scm.org/D470

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