rebase: calculate ancestors for --base separately (issue5420) Previously, the --base option only works with a single "branch" - if there is one changeset in the "--base" revset whose branching point(s) is/are different from another changeset in the "--base" revset, "rebase" will error out with: abort: source is ancestor of destination This happens if the user has multiple draft branches, and uses "hg rebase -b 'draft()' -d master", for example. The error message looks cryptic to users who don't know the implementation detail. This patch changes the logic to calculate the common ancestor for every "base" changeset separately so we won't (incorrectly) select "source" which is an ancestor of the destination. This patch should not change the behavior where all changesets specified by "--base" have the same branching point(s). A new situation is: some of the specified changesets could be rebased, while some couldn't (because they are descendants of the destination, or they do not share a common ancestor with the destination). The current behavior is to show "nothing to rebase" and exits with 1. This patch maintains the current behavior (show "nothing to rebase") even if part of the "--base" revset could be rebased. A clearer error message may be "cannot find branching point for X", or "X is a descendant of destination". The error message issue is tracked by issue5422 separately. A test is added with all kinds of tricky cases I could think of for now.