notify: add maxdiffstat option to truncate long file lists
Large scale changes like a new GCC version can easily result in 1MB+
emails due to diffstat alone. The new maxdiffstat option truncates the
list similar to what maxdiff already provides for the diffs.
Differential Revision: https://phab.mercurial-scm.org/D3402
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors