mercurial/templates/atom/header.tmpl

commit: tell user what to do with .hg/last-message.txt

I have always assumed that the message will be reused by the next `hg
commit`, but it seems it's just silently dropped on the next
commit. Let's try to be more helpful by telling the user that they
have to manually tell hg to reuse it.

The file will still be lost if the user runs some other operation in
between (like a non-in-memory rebase). That will be fixed once we've
switched all operations to be in-memory :)

I didn't include `$(hg root)/` in the path in the message to the user
because that would have made the message too long. Hopefully the user
will figure that part out themselves.

Differential Revision: https://phab.mercurial-scm.org/D8463

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