git-gui: Implemented create branch GUI.

git-gui: Implemented create branch GUI.

Users may now create new branches by activating the Branch->Create menu
item.  This opens a dialog which lets the user enter the new branch
name and select the starting revision for the new branch.

For the starting revision we allow the user to either select from a
list of known heads (aka local branches) or to enter an arbitrary
SHA1 expression.  For either creation technique we run the starting
revision through rev-parse to verify it is valid before trying to
create the ref with update-ref.

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