* test/lisp/vc/vc-git-tests.el (vc-git-test--run): Make sure to
log output from failing Git commands.
(vc-git-test--dir-headers): Stop bothering with vc-dir
internals and just invoke the branch-munging and
header-formatting code we mean to test.
(vc-git-test-dir-branch-headers): Stop invoking vc-dir; just set
default-directory to be able to pass it to the backend function.
15d2fc6498