RFR: git-webrev: add flags --base and --head [v2]

[Erik Helin]

> 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

Erik Helin has updated the pull request with a new target base due to a merge or a rebase. The pull request now
contains two commits:

 - Merge branch 'master'
 - git-webrev: add flags --base and --head

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

Changes: https://git.openjdk.java.net/skara/pull/716/files
 Webrev: https://webrevs.openjdk.java.net/skara/716/webrev.01
  Stats: 29 lines in 1 file changed: 26 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/716.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/716/head:pull/716

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