Fix webkitpy scm_unittests so that they pass again.

We don't run these by default, so they bitrotted at some point.

In particular, it looks like svn 1.7 changed behavior so that if
you call

svn add foo
svn add foo/bar

the second call errors out with bar already having been added;
in svn 1.6, this was just a warning.

This comes up in our code because when adding a file we recursively
add the parent directories to ensure the whole path is versioned; we
need to change this so that we do not implicitly recurse down when
adding a parent directory.

Less importantly, it looks like the semantics for git svn clone -T 
change in Git v2, so the branch names are slightly different and 
we need to account for the remote branch refs differently as a result.

R=eseidel@chromium.org
BUG=

Review URL: https://codereview.chromium.org/397383002

git-svn-id: svn://svn.chromium.org/blink/trunk@178482 bbb929c8-8fbe-4397-9dbb-9b2b20218538