user-manual: Add section "Why bisecting merge commits can be harder ..."

user-manual: Add section "Why bisecting merge commits can be harder ..."

This commit adds a discussion of the challenge of bisecting
merge commits to the user manual.  The original author is
Junio C Hamano <gitster@pobox.com>, who posted the text to
the mailing list <http://marc.info/?l=git&m=119403257315527&w=2>.
His email was adapted for the manual.

The discussion is added to "Rewriting history and maintainig
patch series".  The text added requires good understanding of
merging and rebasing.  Therefore it should not be placed too
early in the manual.  Right after the section on "Problems with
rewriting history", the discussion of bisect gives another reason
for linearizing as much of the history as possible.

The text includes suggestions and fixes by
Ralf Wildenhues <Ralf.Wildenhues@gmx.de> and
Benoit Sigoure <tsuna@lrde.epita.fr>.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>