RFR: git-webrev: use HEAD when branch has no commits

[Erik Helin]

Hi all,

please review this small patch that fixes an issue with the default value for
`--rev` for `git-webrev`. If the current branch has no commits then
`origin..HEAD` will be an empty range, so in that case we should just fall back
to `HEAD`.

Testing:
- Manual testing of `git-webrev` on Linux x64

Thanks,
Erik

-------------

Commit messages:
 - git-webrev: use HEAD when branch has no commits

Changes: https://git.openjdk.java.net/skara/pull/508/files
 Webrev: https://webrevs.openjdk.java.net/skara/508/webrev.00
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/508.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/508/head:pull/508

PR: https://git.openjdk.java.net/skara/pull/508