[BUG] Git-GUI destroys the author name

Since Git version 2.11, git gui destroys the author name when 'git commit amend' is executed and the author name contains UTF-8-specific characters.

This is a known issue from 'Git for Windows' and now a platform-independent bug.

Please refer
- https://github.com/git-for-windows/git/issues/761
- https://github.com/git-for-windows/git/issues/761#issuecomment-220792200
- https://github.com/git-for-windows/git/issues/761#issuecomment-266519859

From my point of view, this is a severe error since the repostory is changed incorrectly. Users of git gui will usually not be able to correct this error with git commands.

Addition to the bug

Proposal https://github.com/git-for-windows/git/commit/30395c645adf21828bbccbdcd3c5c36c39f07050 should be considered and possibly implemented. But as described in https://github.com/git-for-windows/git/issues/761#issuecomment-221057828. And then an implementation without regression is necessary.