filemerge: add `_workingpath` This reduces any reliance on `a`. Differential Revision: https://phab.mercurial-scm.org/D401

filemerge: move a util copy call to filectx.write This way a future in-memory-merge context can intercept them. Differential Revision: https://phab.mercurial-scm.org/D400

filemerge: eliminate most uses of tempfiles Emphasize that they're unused so we can more easily remove them later. Differential Revision: https://phab.mercurial-scm.org/D399

filemerge: extract _maketemp and _makebackup These functions will be modified by in-memory merge, so let's extract them first and add some comments. This also shortens `_filemerge` a bit. Differential Revision: https://phab.mercurial-scm.org/D388

encoding: check overflow while calculating size of JSON escape buffer The minimum input size to exploit is ~682MB (= INT_MAX / len('\\u0000') * 2) on 32bit system, which isn't easy to achieve using Python str in 2GB process address space, but probably doable.

editor: use an unambiguous path suffix for editor files Changes the API of `ui.edit()` to take an optional `action` argument, which is used when constructing the suffix of the temp file. Previously, it was possible to set the suffix by specifying a `suffix` to the optional `extra` dict that was passed to `ui.edit()`, but the goal is to drop support for `extra.suffix` and make `action` a required argument. To this end, `ui.edit()` now yields a `develwarn()` if `action` is not set or if `extra.suffix` is set. I updated all calls to `ui.edit()` I could find in `hg-crew` to specify the appropriate `action`. This means that when creating a commit, instead of the path to the editor file being something like: `/tmp/hg-editor-XXXXXX.txt` it is now something like: `/tmp/hg-editor-XXXXXX.commit.hg.txt` Some editors (such as Atom) make it possible to statically define a [TextMate] grammar for files with a particular suffix. For example, because Git reliably uses `.git/COMMIT_EDITMSG` and `.git/MERGE_MSG` as the paths for commit-type messages, it is trivial to define a grammar that is applied when files of either name are opened in Atom: https://github.com/atom/language-git/blob/v0.19.1/grammars/git%20commit%20message.cson#L4-L5 Because Hg historically used a generic `.txt` suffix, it was much harder to disambiguate whether a file was an arbitrary text file as opposed to one created for the specific purpose of authoring an Hg commit message. This also makes it easier to add special support for `histedit`, as it has its own suffix that is distinct from a commit: `/tmp/hg-histedit-XXXXXX.histedit.hg.txt` Test Plan: Added an integration test: `test-editor-filename.t`. Manually tested: ran `hg ci --amend` for this change and saw that it used `/tmp/hg-editor-ZZjcz0.commit.hg.txt` as the path instead of `/tmp/hg-editor-ZZjcz0.txt` as the path. Verified `make tests` passes. Differential Revision: https://phab.mercurial-scm.org/D464