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
#require docutils gettext
$ $TESTDIR/check-gendoc de
checking for parse errors
Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
warning: please have a space before :hg: