test-notify: make it pass on Mac OS X (again)

Changeset 25e74cd3f023 (from 2008) introduced a hack to handle the
very long values of $TMPDIR typically seen on Mac OS X. This hack
expected continuation lines to begin with a tab. However, as a result
of a change in Python 2.7, changeset 594b98846ce1 made it so
continuation lines began with a tab. Since then, `test-notify' has
been broken on Mac OS X.

Merely replacing the tab in the regular expression with a space will
not work: not only do tab continuations still occur in the message, but
other lines -- in the body of the message -- also start with a
space. Luckily, all broken up lines appear to end with either a colon
or an n, so we can match those, and reinsert them in the replacement.

tests/test-notify

@@ -41,7 +41,7 b' EOF'
 # of the very long subject line
 echo '% pull (minimal config)'
 hg --traceback --cwd b pull ../a 2>&1 |
-  python -c 'import sys,re; print re.sub("\n\t", " ", sys.stdin.read()),' |
+  python -c 'import sys,re; print re.sub("([n:])\\n[\\t ]", "\\1 ", sys.stdin.read()),' |
   sed -e 's/\(Message-Id:\).*/\1/' \
   -e 's/changeset \([0-9a-f]* *\)in .*test-notif/changeset \1in test-notif/' \
   -e 's/^details: .*test-notify/details: test-notify/' \