gitcfg: honor gui.encoding when showing diffs

When no per-file attributes have been set via .gitattributes then we
should respect gui.encoding as the default file encoding.

This allows users to use non-utf-8 file content by setting the
gui.encoding variable, as is described in git-config(1).

Related-to: #303
Suggested-by: Reported-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
doc: update v2.0.4 release notes draft

Signed-off-by: David Aguilar <davvid@gmail.com>
Merge branch 'gui-encoding'

* gui-encoding:
  doc: update v2.0.4 release notes draft
  gitcfg: honor gui.encoding when showing diffs

Closes #303

Signed-off-by: David Aguilar <davvid@gmail.com>