test-encoding: remove support for Git 1.7.x

Git 1.8.0 was released in late 2012. Given that we only support versions of
Mercurial released in 2014, it doesn't make sense to support versions of Git
this old.
git2hg: while extracting metadata, detect which VCS commits were made in

This is currently a heuristic -- it'll be made more reliable in upcoming
patches.
git_handler.import_git_commit: rename 'hg_renames' to 'renames'

In upcoming patches we'll add other sources of renames.
git_handler.import_git_commit: figure out when to detect renames

get_files_changed doesn't do anything with this yet, but it will in upcoming
patches.
git_handler.get_files_changed: switch to diff_tree's tree_changes

object_store.tree_changes doesn't allow us to specify a rename detector, but
diff_tree's tree_changes does.
git_handler: add a way to get a rename detector

This will be used to detect renames in upcoming patches.
git_handler.get_files_changed: return detected renames

We currently return an empty dictionary -- we'll fill it in upcoming patches.