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 symlink
https://bz.mercurial-scm.org/1438
$ hg init
$ ln -s foo link
$ hg add link
$ hg ci -mbad link
$ hg rm link
$ hg ci -mok
$ hg diff -g -r 0:1 > bad.patch
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg import --no-commit bad.patch
applying bad.patch
$ hg status
R link
? bad.patch