Summary:
There are multiple reports that the work in the commit message editor gets lost
for various reasons. We have `.hg/last-message.txt` for commit hook failure,
but that one does not take care of all code paths (ex. metaedit).
This diff changes `ui.edit` directly to try to save messages in `.hg/edit-tmp`
for 2 weeks.
Reviewed By: kulshrax
Differential Revision: D15347831
fbshipit-source-id: 9207adf4315d94a4892685a03f323e89d9c4a7f1
