RFR: 146: git-webrev: Add support for file list argument

[Erik Helin]

Hi all,

please review this pull request that add support for an input argument to `git-webrev`. The argument is a file that itself contains a list of files to include in the webrev. This input file also specifies the order in which the files should appear in the webrev's index.html file.

The main part of the patch is adding support for a list of paths to `ReadOnlyRepository.diff`.

One part of the bug that is not addressed yet is the support for specifying `-` as the input file, `git-webrev` should then read the list of file names (separated by `\n`) from stdin. I plan to address this in a follow-up patch since I wanted to keep this patch as small as possible.

Thanks,
Erik

## Testing
- [x] Added a new unit test
- [x] Manual testing of `git-webrev` with input file
- [x] `make test` passes on Linux x64

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

Commits:
 - e0c3162f: 146: git-webrev: Add support for file list argument

Changes: https://git.openjdk.java.net/skara/pull/275/files
 Webrev: https://webrevs.openjdk.java.net/skara/275/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-146
  Stats: 152 lines in 8 files changed: 146 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/275.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/275/head:pull/275

PR: https://git.openjdk.java.net/skara/pull/275