BlameCache: Add reader for .git-blame-ignore-revs files

Many projects list bulk reformatting and other mechanical commits in a
.git-blame-ignore-revs file at the root of the repository. git (through
blame.ignoreRevsFile), GitHub and GitLab use this file to keep those
commits out of blame. BlameCache.get() already accepts a set of
revisions to ignore, but each caller would have to find, read and parse
the file itself.

Add IgnoreRevFileReader, which reads such a file from the tree of a
commit and returns an IgnoreRevsFile record containing the listed
revisions, ready to pass to BlameCache.get(). Keeping the reader in the
blame cache library lets Gitiles and Gerrit share one implementation.

The format matches git: one full commit SHA-1 per line, "#" starts a
comment and blank lines are skipped. Unlike git, a malformed line is
skipped instead of rejecting the whole file. Only regular files are
read; a directory, symlink or submodule at the path is ignored.

Callers may read the file on every blame request, so reading is
bounded: files larger than 1 MiB are ignored and at most 20,000
revisions are used.

The BlameCache interface is unchanged.

Change-Id: I381018685e49c327c6ba14a5974386535a311913