How do I customize the 'commit message file' generated by `hg commit`?

It's possible to specify this in the [committemplate] config section (see hg help config.committemplate):

 "committemplate"
----------------

"changeset"
    String: configuration in this section is used as the template to
    customize the text shown in the editor when committing.

In addition to pre-defined template keywords, commit log specific one
below can be used for customization:

"extramsg"
    String: Extra message (typically 'Leave message empty to abort
    commit.'). This may be changed by some commands or extensions.

For example, the template configuration below shows as same text as one
shown by default:

  [committemplate]
  changeset = {desc}\n\n
      HG: Enter commit message.  Lines beginning with 'HG:' are removed.
      HG: {extramsg}
      HG: --
      HG: user: {author}\n{ifeq(p2rev, "-1", "",
     "HG: branch merge\n")
     }HG: branch '{branch}'\n{if(activebookmark,
     "HG: bookmark '{activebookmark}'\n")   }{subrepos %
     "HG: subrepo {subrepo}\n"              }{file_adds %
     "HG: added {file}\n"                   }{file_mods %
     "HG: changed {file}\n"                 }{file_dels %
     "HG: removed {file}\n"                 }{if(files, "",
     "HG: no files changed\n")}

"diff()"
    String: show the diff (see 'hg help templates' for detail)

There's no official way to do it w/o modifying mercurial itself (not terribly intimidating, it's very clean Python), but here's a way to do it by tweaking the editor setting the [ui] section of your ~/.hgrc:

editor = hg status --unknown >! /tmp/unknown_list ; /usr/bin/vim -c "r /tmp/unknown_list"

That is, of course vim on Linux specific, but the same could be done for any decent editor on any OS.

I wanted to do this under windows. The idea of customising the editor setting in the ini/.hgrc file made me think of replacing the editor command with a command file.

e.g. if you set this in mercurial.ini:

[ui]
editor = c:\path\to\hgedit.cmd

then hg will call the command file and pass the name of the temp file on the command line. The temp file name can then be accessed in the command file using the %1 parameter.

hgedit.cmd could be something like:

@echo off
hg status --unknown>>%1
notepad %1

If you want to append the output of hg as comments you could do this:

@echo off
echo HG: -->>%1
echo HG: Unknown files:>>%1
for /f "tokens=*" %%a in ('hg st --unknown') do echo HG: %%a>>%1
notepad %1

(You don't have to use notepad of course.)