[PATCH 0 of 3] update: allow crossing branches without -c or -C

[Stuart W Marks]

Patch series to allow updating across branches without -c or -C, round three.

This series consists of three patches:

1. Add a table of 'hg update' cases to comments in merge.py, and add
tests for these cases.  This adds documentation and tests for the
current behavior. This patch has no behavior changes.  Comments added
and tests minimized compared to the previous patch series per request
from mpm.

2. Behavior changes and corresponding test changes to allow crossing of
branches when a specific rev is given, when repo is clean, without
requiring -c or -C.  Branch crossing will abort if no rev is given to
'hg update', per request from mpm.

3. Modify 'hg help update' to reflect the above changes, plus general
cleanup.

See discussion of the previous patch series, starting here:

http://www.selenic.com/pipermail/mercurial-devel/2009-September/015395.html

Please review and comment.

Thanks!

s'marks