RFR: webrev: proper index.html fix and charset fixes

[Erik Helin]

Hi all,

please review this patch that fixes two issues in webrev:

1. Use a more robust band-aid for the problem when generating a webrev for a file named `index.html`
2. Try multiple charsets when reading files from disk

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

Thanks,
Erik

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Commit messages:
 - webrev: proper index.html fix and charset fixes

Changes: https://git.openjdk.java.net/skara/pull/637/files
 Webrev: https://webrevs.openjdk.java.net/skara/637/webrev.00
  Stats: 49 lines in 4 files changed: 35 ins; 2 del; 12 mod
  Patch: https://git.openjdk.java.net/skara/pull/637.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/637/head:pull/637

PR: https://git.openjdk.java.net/skara/pull/637