RFR: git-info: add --upstream flag

[Erik Helin]

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

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Commit messages:
 - git-info: add --upstream flag

Changes: https://git.openjdk.java.net/skara/pull/883/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=883&range=00
  Stats: 13 lines in 1 file changed: 12 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/883.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/883/head:pull/883

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