tests/test-debugrename.t

tests: replace some `python` by `"$PYTHON"` to not only use Python 2

I deleted my `python` binary as a test and a few tests started
failing.

Differential Revision: https://phab.mercurial-scm.org/D10250

  $ hg init
  $ echo a > a
  $ hg ci -Am t
  adding a

  $ hg mv a b
  $ hg ci -Am t1
  $ hg debugrename b
  b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3

  $ hg mv b a
  $ hg ci -Am t2
  $ hg debugrename a
  a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980

  $ hg debugrename --rev 1 b
  b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3