[PATCH 0 of 6] scmutil: enhance case-folding collision detection by casecollisionauditor

[FUJIWARA Katsunori]

This patch series enhances possible case-folding collision detection
at "hg add" by "scmutil.casecollisionauditor".

Before this series, possible case-folding collisions like below can't
be detected at "hg add":

  - adding new "foo" against existing "Foo/bar"
  - adding new "foo/bar" against existing "Foo"
  - adding new "foo/x" against existing "Foo/y"

Difference between V1 and V2:

  - introduce "_oncollision()" (patch #2)

  - delay elimination of removed files from file list given from
    dirstate for efficiency on large manifests

    patch #3 is also part of this revise

Even though patch #1 was replied by Matt as "queued for default", this
series still includes it for consistency of this series, because that
patch doesn't yet appear in master or crew repositories for Mercurial.