RFR: 757: git webrev with file list fails if any file has not changed

[Erik Helin]

Hi all,

please review this patch that makes `git-webrev` print a warning when it encounters a file not present in the diff (for example if the user supplied a file list as argument).

Testing:
- [x] Manual testing on Linux x64

Thanks,
Erik

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Commit messages:
 - skara-757

Changes: https://git.openjdk.java.net/skara/pull/1074/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1074&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-757
  Stats: 14 lines in 3 files changed: 12 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/1074.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1074/head:pull/1074

PR: https://git.openjdk.java.net/skara/pull/1074