git-gui: Cleanup handling of the default encoding.

git-gui: Cleanup handling of the default encoding.

- Make diffs and blame default to the system (locale)
  encoding instead of hard-coding UTF-8.
- Add a gui.encoding option to allow overriding it.
- gitattributes still have the final word.

The rationale for this is Windows support:

1) Windows people are accustomed to using legacy encodings
   for text files. For many of them defaulting to utf-8
   will be counter-intuitive.
2) Windows doesn't support utf-8 locales, and switching
   the system encoding is a real pain. Thus the option.

This patch also adds proper encoding conversion to Apply Hunk/Line.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Tested-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>