commit: don't use hard-coded `.hg/last-message.txt` path in error message
Whoever wrote D8463 just used a hard-coded path to
`.hg/last-message.text` instead of using the relative path that was
already available in the `msgfn` variable (and used just a few lines
up in related message). Let's fix that.
Differential Revision: https://phab.mercurial-scm.org/D12585
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