.arcconfig

fix: fix handling of merge commits by using overlayworkingctx

Most of this code was conceptually copied from what rebase does, with one small
difference: hgext.rebaserev.rebase uses branchmerge=True, while I had to use
branchmerge=False, or else it got really confused about updating to the same
revision in some situations. I believe that the difference is that rebase is
always dealing with *some* form of update - it never gets to mergemod.update if
the source and destination are the same, while we can encounter that situation
with fix. This may imply that this code has some issues with named branches that
should be investigated.

Differential Revision: https://phab.mercurial-scm.org/D7703

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