RFR: git-webrev: add flags --base and --head

[Robin Westberg]

On Sat, 15 Aug 2020 10:42:37 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> please review this patch that adds two new flags to `git-webrev` - `--base` and `--head`. The flag `--base` behaves
> like `-r, --rev`, that is determines the base commit to compare against. The flag `--head` determines the commit to use
> a `HEAD` for the comparison, that is the most recent commit for the comparison. The two flags `--base` and `--head`
> allows a user to create a webrev between two arbitrary commits (without forcing to check out the head commit first).
> Testing:
> - Manual testing on Linux x64
> 
> Thanks,
> Erik

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/716