abort: added support for merge This adds support of `hg merge --abort` to `hg abort` plan. This involves refactoring `hg.merge` into two different functions removing the abort logic of `merge` from `hg.merge` and then creating a seperate `hg.abortmerge` to handle the abort logic so that the abortion of merge can be called independently. `hg.abortmerge` is then registered as `abortfunc` for the state detection API so that `commands.abort` can use it to deal with an unfinished merge operation. Results are shown as tests. Differential Revision: https://phab.mercurial-scm.org/D6588