[PATCH 0 of 3 STABLE V3] context: add 'dirs()' to changectx/workingctx for directory pattern

[FUJIWARA Katsunori]

this patch series adds 'dirs()' to changectx/workingctx for directory
patterns.

patch #2 is a bug fix for 'hg status' with directory patterns and the
target revision in which files unknown for working context are tracked
under the pattern matched sub directory.

patch #3 is a kind of refactoring for invalid pattern detection in
'changectx.walk()'.