git-gui: Fix post-commit status with subject in non-locale encoding

git-gui: Fix post-commit status with subject in non-locale encoding

As pointed out in msysgit bug #181, when a non-locale encoding is
used for commits, post-commit status messages display the subject
incorrectly.  It happens because the file handle is not properly
configured before the subject is read back.

This patch fixes it by factoring out the code that is used to setup
the output handle into a separate function, and calling it from
the reading code.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Acked-by: Robin Rosenberg <robin.rosenberg@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>