Should notify when git diff would prevent publish

[dillonkearns]

elm publish will abort if there are any uncommitted changes, using this logic:

https://github.com/elm/compiler/blob/770071accf791e8171440709effe71e78a9ab37c/terminal/src/Publish.hs#L257-L264

elm-publish-action should be as close as possible to giving the elm publish feedback as a dry run when it is running without a new version to publish. The goal should be to have as much confidence as possible that the publish will succeed if and when you bump the version.

Auto stash option

As a bonus, it might be convenient to also provide a stash option. To be explicit, the default should be to give an error if there are uncommitted changes. But if you want to ignore them, there could be an option to automatically stash and unstash to run the command.