merge: let _forgetremoved() work on the file->action dict By moving the conversion from the file->action dict after _forgetremoved(), we make that method shorter by removing the need for the confusing 'xactions' variable.

merge: let _resolvetrivial() work on the file->action dict By moving the conversion from the file->action dict after _resolvetrivial(), we greatly simplify and clarify that method.

merge: let bid merge work on the file->action dict By moving the conversion from the file->action dict after the bid merge code, bid merge can be simplified a little. A few tests are affected by this change. Where we used to iterate over the actions first in order of the action type ('g', 'r', etc.) [1], we now iterate in order of filename. This difference affects the order of debug log statements. [1] And then in the non-deterministic order of files in the manifest dictionary (the order returned from manifest.diff()).

merge: write manifestmerge() using dictionary with entry per file In the same vein as 38e55e55ae4d (largefiles: rewrite merge code using dictionary with entry per file, 2014-12-09), rewrite manifestmerge() itself as dictionary with the filename as key. This will let us simplify some of the other code in merge.py and eventually drop the conversion in the largefiles code. No difference in speed could be detected (well within the noise level when run in Mozilla repo).