RFR: Use positional argument if present

[Leo Korinth via github.com]

I hope this implements what is documented in the help message:
```usage: git webrev [options] [<rev>]```

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Commits:
 - fb4660f0:	Use positional argument if present

Pull request:
https://git.openjdk.java.net/skara/pull/75

Webrev:
https://webrevs.openjdk.java.net/skara/75/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/75.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/75/head:pull/75