RFR: forge: rename HostedCommitMetadata to HostedCommit

[Erik Helin]

Hi all,

please review this patch that refactors `HostedCommitMetadata` to `HostedCommit`. The changes are primarily in fetching
the diff for the commit in addition to the metadata already being fetched.

Thanks,
Erik

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Commit messages:
 - forge: rename HostedCommitMetadata to HostedCommit

Changes: https://git.openjdk.java.net/skara/pull/879/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=879&range=00
  Stats: 287 lines in 7 files changed: 174 ins; 105 del; 8 mod
  Patch: https://git.openjdk.java.net/skara/pull/879.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/879/head:pull/879

PR: https://git.openjdk.java.net/skara/pull/879