If for whatever reasons a repo already had a `release-tools` directory
before doing a clean import of it with `git subtree`, the check used
to fail because it found those old commits.

This can be fixed by telling `git log` to stop when the directory
disappears from the repo. There has to be a commit with removes the
old content, because otherwise `git subtree add` doesn't work.

Fixes: https://github.com/kubernetes-csi/external-resizer/issues/21

test enhancements

build.make: fix pushing of "canary" image from master branch

The introduction for each individual test looked like an actual
command:

  test-subtree
  ./release-tools/verify-subtree.sh release-tools
  Directory 'release-tools' contains non-upstream changes:
  ...

It's better to make it look like a shell comment and increase its
visibility with a longer prefix:

  ### test-subtree:
  ./release-tools/verify-subtree.sh release-tools
  ...

After merging into external-attacher, the next Travis CI run did not
push the "canary" image because the check for "canary" only covered
the case where "-canary" is used as
suffix (https://travis-ci.org/kubernetes-csi/external-attacher/builds/484095261).

Individual repos may have to filter out certain packages from
testing. For example, in csi-test the cmd/csi-sanity directory
contains a special test that depends on additional parameters that set
the CSI driver to test against.

This may or may not work, depending on which packages have tests and
whether they contain glog.

check subtree for changes

We don't want to allow local modifications in the subtree. Everything
should go to the csi-release-tools repo first.

"make test" used to abort after the first test failure. That was
partly intentional: if the simple tests already fail (for example,
because of a syntax error), then there is no point in continuing to
test.

However, it also makes it harder to find all errors in a CI system
when the errors are unrelated (first error shows up, gets fixed, next
error shows up, etc.).

Now "make test" still aborts early, but "make -k test" is used in the
CI and will run all individual tests because they are split up into
different targets.

The goal is to enforce that changes get merged upstream first and only
get into the local repo via a normal "git subtree merge".

README.md: fix repo URL for initial setup

Copy-and-paste error from the time when the
kubernetes-csi/csi-release-tools repo didn't have the code...

initial content

The actual repository was not named like the prototype repo.

It's worth calling out explicitly that only the master branch is
maintained.

The repo was created with an HTML version of the build.make file from
https://github.com/pohly/csi-build-rules/. Here's the raw file.