git-gui: View blame from the command line.

git-gui: View blame from the command line.

Viewing annotated files is one of those tasks that is relatively
difficult to do in a simple vt100 terminal emulator.  The user
really wants to be able to browse through a lot of information,
and to interact with it by navigating through revisions.

Now users can start our file viewer with annotations by running
'git gui blame commit path', thereby seeing the contents of the
given file at the given commit.  Right now I am being lazy by
not allowing the user to omit the commit name (and have us thus
assume HEAD).

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