bash_completion: add global support for -b|--branch

Previously, only -r|--rev was parsed globally which meant 'hg push -b <tab>'
would try to complete a path instead of a branch.

contrib/bash_completion

@@ -255,6 +255,13 b' shopt -s extglob'
                 fi
                 return 1
             ;;
+            -b|--branch)
+                if [[ $canonical = 1 || status != "$cmd"* ]]; then
+                    _hg_branches
+                    return 0
+                fi
+                return 1
+            ;;
         esac
     fi