Added --ignore-file argument, so you can specify ignored commits in a
file rather than as raw command-line arguments. Also, automatically
searches for a file called .git-blame-ignore-revs, which is
automatically used as an ignore list by default.

Also, specifying an unknown revision (either on the command line or in a
file) now generates a warning, not an error.

Notes on some decisions:
- The file is called .git-blame-ignore-revs (not mentioning hyper-blame)
  because we may use the same list in tools other than hyper-blame in
  the future.
- We look at the *currently checked out* version of
  .git-blame-ignore-revs (not the version at the specified revision) for
  consistency with .git-ignore. Because we only expect revisions to be
  added (not deleted), it should be fine to use an ignore list from a
  newer version than the revision being blamed.
- We considered using git notes for the ignore list so that you could
  add a revision to the ignore list without needing a follow-up CL.
  However, there are some problems with this approach. git notes is not
  automatically synced with git clone/pull. Also the Chromium infra
  tools (Reitveld, CQ) are not set up to allow modification of git
  notes, nor are changes to git notes subject to OWNERS checks. Using a
  regular file ensures all users synced to a particular revision are
  using the same ignore list.

BUG=574290

Review URL: https://codereview.chromium.org/1697423004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@298897 0039d316-1c4b-4281-b951-d872f2087c98