git-gui: Show only the abbreviated SHA1 after committing.

git-gui: Show only the abbreviated SHA1 after committing.

There's really no great reason to show the entire commit object id
within the GUI, especially if the user is unable to copy and paste
it into another interface such as gitk or a terminal window.  So
we'll just show them the first 8 digits and hope that is unique
within their repository.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>