git-gui: Use gitattribute "encoding" for file content display

git-gui: Use gitattribute "encoding" for file content display

Most folks using git-gui on internationalized files have complained
that it doesn't recognize UTF-8 correctly.  In the past we have just
ignored the problem and showed the file contents as binary/US-ASCII,
which is wrong no matter how you look at it.

This really should be a per-file attribute, managed by .gitattributes,
so we now pull the "encoding" attribute data for the given path from
the .gitattributes (if available) and use that, falling back to UTF-8
if the attributes are unavailable, git-check-attr is broken, or an
encoding for this path not specified.

We apply the encoding anytime we show file content, which currently
is limited to only the diff viewer and the blame viewer.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>