RFR: git-webrev: add --similarity flag

[Erik Helin]

Hi all,

please review this patch that adds the `--similarity` flag to `git-webrev`. The `--similarity` flag allows users to
manually tune the threshold for when a file should be considered renamed or copied.

Testing:
- Manual testing on Linux x64

Thanks,
Erik

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Commit messages:
 - git-webrev: add --similarity flag

Changes: https://git.openjdk.java.net/skara/pull/715/files
 Webrev: https://webrevs.openjdk.java.net/skara/715/webrev.00
  Stats: 69 lines in 6 files changed: 46 ins; 0 del; 23 mod
  Patch: https://git.openjdk.java.net/skara/pull/715.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/715/head:pull/715

PR: https://git.openjdk.java.net/skara/pull/715