Prepopulate commit message buffer with hg's usual output

[Wilfred]

This seems to work if you have with-editor installed:

M-x eshell
$ with-editor-export-editor                                                                                                                                                                                                                                                   
Successfully exported EDITOR
$ hg commit

This give you a nice buffer with HG things:

HG: Enter commit message.  Lines beginning with 'HG:' are removed.
HG: Leave message empty to abort commit.
HG: --
HG: user: Wilfred Hughes <me@wilfred.me.uk>
HG: changed ...

[Wilfred]

It'd be worth mentioning C-c C-c too, so users know how to exit the new commit message buffer.

[Bounga]

@Wilfred Yes and C-c C-k too.