editor: file created for diff action should have .diff suffix This is a follow-up to https://phab.mercurial-scm.org/D464 (6e6452bc441d) that introduced the new file extension behavior. It erroneously changed `.diff` to `.diff.hg.txt`. Test Plan: Verified `make tests` passes, particularly `test-editor-filename.t`. Differential Revision: https://phab.mercurial-scm.org/D607