git-gui: Describe deleted symlinks in a more friendly way.

git-gui: Describe deleted symlinks in a more friendly way.

Currently core-git's diff utilities report a deleted symlink as a
deleted file with a mode of 120000.  This is not nearly as user
friendly as one might like, as the user must remember that 120000
is the UNIX mode bits for a symlink.  So instead we transform
the not-so-friendly message from core-git into a slightly more
user friendly "deleted symlink" message.

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