pr-upload: add --no-commit flag

[Rylan12]

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Allow pr-upload to run without committing changes

This is useful in a few special cases (such as Homebrew/homebrew-core#62022) where the bottle upload failed but the bottle commit made it into master. Now, running brew pr-upload --no-commit in the bottle directory will upload the bottles without trying to generate a new bottle commit.

The current alternative is to check out a commit before the bottle commit, run pr-pull, and then reset to origin/master. This situation is rare enough that using that workflow isn't terrible, but here's a potential quick fix to make it even easier.

CC @jonchang

[jonchang]

LGTM. Thanks for updating the docs too!