Branch b1.
  
    $ pg branch b1
    $ pg checkout b1
    Switched to branch 'b1'
    $ cat > $PLUGIN_DIR/aliases.zsh <<EOF
    > alias hehe='echo hehe from b1'
    > EOF
    $ pg commit -am 'Change for b1'
    \[b1 [a-f0-9]{7}\] Change for b1 (re)
     1 file changed, 1 insertion(+), 1 deletion(-)
  
  Go back to master.
  
    $ pg checkout master
    Switched to branch 'master'
  
  Load plugin from b1.
  
    $ antigen-bundle $PLUGIN_DIR --branch=b1 &> /dev/null
    $ hehe
    hehe from b1
  
  Does not say 'Already on b1' on each session startup.
  
    $ antigen-bundle $PLUGIN_DIR --branch=b1
  
  Load plugin from master.
  
    $ antigen-bundle $PLUGIN_DIR &> /dev/null
    $ hehe
    hehe