Add script that reindents recently changed files.

Add script that reindents recently changed files.

My laptop needs about 30s to run clang-format since the script currently
reindents all files regardless of whether or not they have been
modified. This PR adds a new target indent-branch that only reindents
files that have been modified (or added) since the last commit to
master. This takes less than a second to run (with a few files changed)
on my laptop.