git-debrebase: test suite: gdr-viagit: new test

git-debrebase: test suite: gdr-viagit: new test

Have verified with this:

  diff --git a/tests/tests/gdr-viagit b/tests/tests/gdr-viagit
  index 55fe4d74..20576c23 100755
  --- a/tests/tests/gdr-viagit
  +++ b/tests/tests/gdr-viagit
  @@ -17,6 +17,7 @@ git-debrebase)	;;
	  cat <<'END' >$st
   #!/bin/sh
   set -x
  +export GIT_DIR=$PWD/`git-rev-parse --git-dir`
   exec "$DGIT_GITDEBREBASE_TEST_REAL" "$@"
   END
	  chmod +x $st

that this would indeed spot at least some problems.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>