[PATCH 0 of 2] scmutil: make filename examination extensible

[FUJIWARA Katsunori]

This patch series makes filename examination extensible.

This allows extensions to customize filename examination easily: for
example, limiting normalization of filenames to NFC or NFD for
cooperation between MacOS and other platforms.

In patch #2, the loop by "existingiter" is driven not in the
constructor of each checkers, but in the constructor of
"namecollisionauditor", because it can't be used for loop twice or
more, if it is really iterator.

But it is fact that "casecollisionauditor" is used only with "wctx" in
current Mercurial core code. So, if we can recognize it not as
"existingiter" but as "wctx", initialization of each checkers can be
encapsulated into themselves and be improved in performance by the way
proposed by Joshua Redstone.

    http://www.selenic.com/pipermail/mercurial-devel/2012-July/042527.html

Should I rename "existingiter" to "wctx" ?