Update "crosses branches"

[Stephen Rasku]

I issued a "hg up -r tip" to get to the tip of my repository.  When I
issue that command, I get the following error:

    abort: crosses branches (use 'hg merge' or 'hg update -C')

"hg status" confirms that there are no changed files.  Is it expected
to use "hg update -C" even when there is nothing modified?  There is
nothing in "hg help update" about using -C when there are no
modifications.  Using the -C option resolved the issue but I am
confused as to why it was necessary.

This is for Mercurial 1.2.1.

...Stephen