llvm_deps_bump: Write "change" not "changes" when there's only one change
llvm_deps_bump: Add --list option for listing new LLVM changes only

This stops the tool from creating a new branch and Rietveld review.
llvm_deps_bump: Skip over "BUG=None" as well as "BUG=none" in commit message
llvm_deps_bump: Add partial support for updating 'pnacl/git/gcc' revision

This doesn't work fully: References to origin/master and
origin/upstream/master don't work with this GCC branch.