Make release script check if local gutenberg-mobile branch is up-to-date

[mchowning]

The release script checks that we are releasing from an appropriate branch (like develop), but it does not verify that the local branch is up-to-date. This resulted in some commits accidentally getting left out of a recent release (not included in 1.44.0, added in 1.44.1).

It's really easy to forget to update the local branches before running the release script (I'm surprised this is the first time this has bitten us), so let's have the release script verify that the current gutenberg-mobile branch is up-to-date with the remote repository before allowing the release process to proceed.

cc: @guarani , @cameronvoell