emacs: Avoid infinite loop when marking up citations.

Thanks to Dirk Hohndel for reporting the bug. The infinite loop was first
noticed in the following message (available from the Linux kernel mailing list):

	alpine.LFD.2.00.0912081304070.3560@localhost.localdomain

Note that the bug does not show up when viewing the message in
isolation---the bug was triggered only when viewing this file indented
to a depth of at least 13.

The fix is simply to use a marker rather than an integer position when
recording a point we plan to move back to later, (since inserting the
indented button causes the buffer position of the desired marker to
change).