histedit: save manually edited commit message into ".hg/last-message.txt"
Before this patch, manually edited commit message for "message"
command in histedit-ing is not saved into ".hg/last-message.txt" until
it is saved by "localrepository.savecommitmessage()" in
"localrepository.commit()".
This may lose such commit message, if unexpected exception is raised.
This patch saves manually edited commit message for "message" comand
in histedit-ing into ".hg/last-message.txt" just after user editing.
This is the simplest implementation to fix on stable. Editing and
saving commit message should be centralized into the framework of
"localrepository.commit()" with "editor" argument in the future.
This patch uses repository wrapping class for exception raising before
saving commit message in "localrepository.commit()" easily and
certainly, because such exception requires corner case condition.