From the hgrc docs section on "hooks" -
"incoming"
Run after a changeset has been pulled, pushed, or unbundled into the
local repository. The ID of the newly arrived changeset is in
"$HG_NODE". URL that was source of changes came is in "$HG_URL".
or...
"post-<command>"
Run after successful invocations of the associated command. The contents
of the command line are passed as "$HG_ARGS" and the result code in
"$HG_RESULT". Parsed command line arguments are passed as "$HG_PATS" and
"$HG_OPTS". These contain string representations of the python data
internally passed to <command>. "$HG_OPTS" is a dictionary of options
(with unspecified options set to their defaults). "$HG_PATS" is a list
of arguments. Hook failure is ignored.
(The documentation also goes into detail about what the config should actually look like and how hook scripts are called.)