Improve squash merge commit author and co-author with private emails (#22977)

When emails addresses are private, squash merges always use
`@noreply.localhost` for the author of the squash commit. And the author
is redundantly added as a co-author in the commit message.

Also without private mails, the redundant co-author is possible when
committing with a signature that's different than the user full name and
primary email.

Now try to find a commit by the same user in the list of commits, and
prefer the signature from that over one constructed from the account
settings.

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