help/environment.txt

commit: save commit message so it's not destroyed by rollback.
(issue1635)

Rationale: if a pretxncommit hook rejects this commit, the transaction
is rolled back and the user's commit message is irrevocably lost.
So save a copy in .hg/last-message.txt, just in case.  Also handy if
the user deliberately rolls back a commit in order to amend it.

HG
    Path to the 'hg' executable, automatically passed when running
    hooks, extensions or external tools. If unset or empty, this is
    the hg executable's name if it's frozen, or an executable named
    'hg' (with %PATHEXT% [defaulting to COM/EXE/BAT/CMD] extensions on
    Windows) is searched.

HGEDITOR
    This is the name of the editor to run when committing. See EDITOR.

    (deprecated, use .hgrc)

HGENCODING
    This overrides the default locale setting detected by Mercurial.
    This setting is used to convert data including usernames,
    changeset descriptions, tag names, and branches. This setting can
    be overridden with the --encoding command-line option.

HGENCODINGMODE
    This sets Mercurial's behavior for handling unknown characters
    while transcoding user input. The default is "strict", which
    causes Mercurial to abort if it can't map a character. Other
    settings include "replace", which replaces unknown characters, and
    "ignore", which drops them. This setting can be overridden with
    the --encodingmode command-line option.

HGMERGE
    An executable to use for resolving merge conflicts. The program
    will be executed with three arguments: local file, remote file,
    ancestor file.

    (deprecated, use .hgrc)

HGRCPATH
    A list of files or directories to search for hgrc files. Item
    separator is ":" on Unix, ";" on Windows. If HGRCPATH is not set,
    platform default search path is used. If empty, only the .hg/hgrc
    from the current repository is read.

    For each element in HGRCPATH:

    - if it's a directory, all files ending with .rc are added
    - otherwise, the file itself will be added

HGUSER
    This is the string used as the author of a commit. If not set,
    available values will be considered in this order:

    - HGUSER (deprecated)
    - hgrc files from the HGRCPATH
    - EMAIL
    - interactive prompt
    - LOGNAME (with ``@hostname`` appended)

    (deprecated, use .hgrc)

EMAIL
    May be used as the author of a commit; see HGUSER.

LOGNAME
    May be used as the author of a commit; see HGUSER.

VISUAL
    This is the name of the editor to use when committing. See EDITOR.

EDITOR
    Sometimes Mercurial needs to open a text file in an editor for a
    user to modify, for example when writing commit messages. The
    editor it uses is determined by looking at the environment
    variables HGEDITOR, VISUAL and EDITOR, in that order. The first
    non-empty one is chosen. If all of them are empty, the editor
    defaults to 'vi'.

PYTHONPATH
    This is used by Python to find imported modules and may need to be
    set appropriately if this Mercurial is not installed system-wide.