Specifying git commit doesn't work if plugin is already installed

Off the back of https://github.com/wbthomason/packer.nvim/issues/200#issuecomment-777848523 I just discovered and used the commit key which is a really nice feature, my initial attempt involved just adding a specific commit to an already installed plugin which resulted in an invalid commit error. Specifying the full commit not just the first characters resulted in an even more obscure git tree error. Finally deleting the plugin and re-installing it with the key specified did work.

Ideally just adding a commit to an already installed plugin as a key would be enough

PS: apologies think I'm averaging an issue a day at this rate 😓