Require git 1.5.4 or later in Build.PL

  I've decided that 1.5.4 will be the minimum supported version,
  because Git::Check needs the --exclude-standard option, which
  didn't exist until then.  There are other issues with old Git
  versions, and I'm tired of CPAN testers reports because of them.
In tests, restore original cwd only if saved

  If a test bails out with skip_all (or aborts for some other reason)
  before saving cwd, we don't need warnings about chdir(undef).
v2.015

 - bail out of Build.PL unless git 1.5.4 or later is found (christopher madsen)
 - prevent warnings about chdir(undef) in tests (christopher madsen)