Fix `--update-message` option of `spr diff` when commit is unchanged

[sven-of-cord]

@HenryDavies reported that the --update-message option of spr diff does not work. It looks like it only works when there were changes to the pull request (as in, the commit) to be submitted. When the user only changes the commit message, but makes no changes to the code, spr diff --update-message should still update the message on GitHub (but not submit a new commit to the pull request branch). This commit fixes this. The easiest way to fix this is to check whether --update-message was given when we do the early exit ("No update needed"), and then just update the message. This should be easier then getting rid of the early exit and then changing all the code below to not produce and push a commit, just so that the pull request update code that we already have (and that does update the message) is not skipped. I moved some of the code into a new method on PullRequestUpdate to reduce code duplication.

Test Plan:

• submit this commit as pull request with an empty summary
• add the summary to the local commit's message
• run spr diff --update-message
• check that message was updated on GitHub
• add a line to the CHANGELOG for this fix, then update the local commit message, run spr diff --update-message again to see that it updates both PR contents and commit message on GitHub

[sven-of-cord]

Test plan completed. It worked nicely.