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
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