tests: better test coverage of merges of flags This makes existing problems with merges of symlinks and files more obvious and add test coverage for tricky merges without real common ancestors.

merge: remove old pre-audit code checking for absolute paths Audit will handle this in a more elegant way.

merge: drop reference to file contents immediately after write Like 7d2aaeea67ed this reduces memory usage on large merges.

merge: changing the mode of a file is also an update It was a change in the file system that wasn't reported in the summaries.

merge: use util.unlinkpath for removing moved files - more like how removed files are removed.

merge: .hgsubstate is special as merge destination, not as merge source

merge: remove redundant unlink after merge The early prescan for move/remove and removal of moved files in applyupdates was introduced with mergestate 368a4ec603cc and rendered this chunk of code irrelevant. The impact of the chunk was reduced in 5b3383ea67d2 - but it could have been removed completely.

merge: rename list of actions from action to actions

merge: consistently use "x" instead of 'x' for internal action types This makes it simpler to search for places where the action types are handled.

merge: consistently use repo.wopener.audit instead of creating a new auditor