Load and test plugin.
  
    $ antigen-bundle $PLUGIN_DIR
    Cloning into '.+?'\.\.\. (re)
    done.
    $ hehe
    hehe
  
  Save the current HEAD of the plugin.
  
    $ old_version="$(pg rev-parse HEAD)"
  
  Modify the plugin.
  
    $ cat > $PLUGIN_DIR/aliases.zsh <<EOF
    > alias hehe='echo hehe, updated'
    > EOF
    $ pg commit -am 'Updated message'
    \[master [a-f0-9]{7}\] Updated message (re)
     1 file changed, 1 insertion(+), 1 deletion(-)
  
  Save the new HEAD of the plugin.
  
    $ new_version="$(pg rev-parse HEAD)"
  
  Define a convenience function to get the current version.
  
    $ current-version () {(cd dot-antigen/repos/* && git rev-parse HEAD)}
  
  Confirm we currently have the old version.
  
    $ [[ $(current-version) == $old_version ]]
  
  Run antigen's update.
  
    $ antigen-update
    **** Pulling */test-plugin (glob)
    From */test-plugin (glob)
       ???????..???????  master     -> origin/master (glob)
    Updating ???????..??????? (glob)
    Fast-forward

     aliases.zsh |\s+2 \+- (re)

     1 file changed, 1 insertion(+), 1 deletion(-)
    Updated from ??????? to ???????. (glob)
    ??????? Updated message (glob)

     aliases.zsh |\s+2 +- (re)

     1 file changed, 1 insertion(+), 1 deletion(-)
    
  
  Confirm we have the new version.
  
    $ [[ $(current-version) == $new_version ]]
  
  Run update again, with no changes in the origin repo.
  
    $ antigen-revert
    Reverted to state before running -update on *. (glob)
  
  Confirm we have the old version again.
  
    $ [[ $(current-version) == $old_version ]]