branch: allow changing branch name to existing name if possible With the functionality added in previous patch we can change branches of a revision but not everytime even if it's possible to do so. For example cosider the following case: o 3 added a (foo) o 2 added b (foo) o 1 added c (bar) o 0 added d (bar) Here if I want to change the branch of rev 2,3 to bar, it was not possible and it will say, "a branch with same name exists". This patch allows us to change branch of 2,3 to bar. The underlying logic for changing branch finds the changesets from the revs passed which have no parents in revs. We only support revsets which have only one such root, so to support this we check whether the parent of the root has the same name as that of the new name and if so, we can use the new name to change branches. Differential Revision: https://phab.mercurial-scm.org/D1913