git-gui: fix initial git gui message encoding
This fix refers https://github.com/git-for-windows/git/issues/664
After `git merge --squash` git creates .git/SQUASH_MSG (UTF-8 encoded)
which contains squashed commits. When run `git gui` it copies SQUASH_MSG
to PREPARE_COMMIT_MSG, but without honoring UTF-8. This leads to encoding
problems on `git gui` commit prompt.
The same applies on git cherry-pick conflict, where MERGE_MSG is created
and then is copied to PREPARE_COMMIT_MSG.
In both cases PREPARE_COMMIT_MSG must be configured to store data in UTF-8.
Signed-off-by: yaras <yaras6@gmail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>