github #119: fix completion for git bisect visualize

When trying to use the completion for git bisect visualize one currently
is greeted with the curious error of something like this:

    git bisect visualize _git-bisect:89: command not found: arch

While in this case 'arch' is the name of the first folder in the
directory. The cause for this is a missing separation of switch cases
which we fix by inserting the needed ";;".

Signed-off-by: Christian Heusel <christian@heusel.eu>

Completion/Unix/Command/_git

@@ -278,6 +278,7 @@ _git-bisect () {
           _arguments -C -s \
             $log_options \
             $revision_options && ret=0
+          ;;
         (*)
           _nothing
           ;;