commit: save commit message so it's not destroyed by rollback.
(
issue1635)
Rationale: if a pretxncommit hook rejects this commit, the transaction
is rolled back and the user's commit message is irrevocably lost.
So save a copy in .hg/last-message.txt, just in case. Also handy if
the user deliberately rolls back a commit in order to amend it.
#!/bin/sh
hg init
touch a
hg add a
hg ci -m "a" -d "1000000 0"
echo 123 > b
hg add b
hg diff --nodates
hg diff --nodates -r tip
echo foo > a
hg diff --nodates
hg diff -r ""
hg diff -r tip -r ""
true