Copy git-filter-branch from git 1.9.3 (unchanged)

We need to change this script to incorporate --date-order. Further
changes will be needed for running it externally.
Allow running git-filter-branch externally

We need to find the git support functions, which usually are just in the
samed directory as the rest.
Move --date-order into git-filter-branch

It was possible to specify --date-order on git-filter-branch's command
line, but this was broken in
git/git@3361a548dbedde96d75bd4134e9ab9e6d82774dd.
Mention hacked git-filter-branch in README
Merge pull request #1 from Blaisorblade/topic/date-order

Add copy of git-filter-branch with --date-order in to allow usage