icasefs: use util.normcase() instead of str.lower() or os.path.normpath()

merge with stable

test-subrepo: test for Issue3153

diff: don't crash when diffing a revision with a deleted subrepo (issue3153) When a user requested a diff between a revision (r1) that contained a subrepo and another (r2) that did not, mercurial would crash if r1 was specified before r2 but would execute the diff otherwise. This fixes this behavior by skipping the missing subrepo in the diff.

status: add missing ":" to help text

dispatch: remove extra newline The return statement belongs to the lines above.