mail.sh: support MAIL_SH_OTHER_REVISIONS environment variable
If MAIL_SH_OTHER_REVISIONS is set and a single update is being
processed passed in via command line arguments ($1 $2 $3) then
when running show_new_revisions, mail.sh will use the contents
of MAIL_SH_OTHER_REVISIONS as the list of revision tips that
have already been seen rather than making its own guess at what
those might be.
This allows a caller to do smarter processing which can result
in show_new_revisions producing a more accurate list of new
revisions that have never been seen before.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>