CONTRIBUTING

resolve: add option to warn/abort on -m with unresolved conflict markers

When a user is dropped out of Mercurial to a terminal to resolve files, we emit
messages like:

    conflicts while merging file1! (edit, then use 'hg resolve --mark')
    conflicts while merging file2! (edit, then use 'hg resolve --mark')

We don't mention a file name in the hint, so some users might do something like
`$EDITOR file1; hg resolve --mark`, see that it says "(no more unresolved
files)" and forget to deal with file2 before running the next command.

Even if we did mention a file name in the hint, it's too easy to forget it
(maybe the merge spans a couple days or something). This option lets us inform
the user that they might have missed something.

In the scenario above, the output would be something like:

    warning: the following files still have conflict markers:
      file2
    (no more unresolved files)

Differential Revision: https://phab.mercurial-scm.org/D4035

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