RFR: git-info: add --upstream flag

[Robin Westberg]

On Wed, 14 Oct 2020 12:32:01 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> please review this patch that adds the `--upstream` flag to `git-info` to show the hash for the original (upstream)
> commit that that provided commit was backported from. For example, if commit `B` has been backported from commit `A`,
> then a user can run:  $ git info --upstream B
> A
> 
> Testing:
> - [x] Manual testing on Linux x64
> 
> Thanks,
> Erik

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/883