merge: report all files in _checkunknown
When doing hg up, if there is a file conflict with untracked files,
currently only the first such conflict is reported. With this patch,
all of them are listed.
With this patch error message is now reported as
a: untracked file differs
b: untracked file differs
abort: untracked files in working directory conflict with files in
requested revision
instead of
abort: untracked file in working directory differs from file in
requested revision: 'a'
This is a follow up to an old attempt to do this here:
http://selenic.com/pipermail/mercurial-devel/2011-August/033625.html