Tests for core subproject support

Tests for core subproject support

The following tests available:

- create subprojects: create a directory in the superproject,
  initialize a git repo in it, and try adding it in super project.
  Make a commit in superproject

- check if fsck ignores the subprojects: it just should give no errors

- check if commit in a subproject detected: make a commit in
  subproject, git-diff-files in superproject should detect it

- check if a changed subproject HEAD can be committed: try
  "git-commit -a" in superproject. It should commit changed
  HEAD of a subproject

- check if diff-index works for subproject elements: compare the index
  (changed by previuos tests) with the initial commit (which created
  two subprojects). Should show a change for the recently changed subproject

- check if diff-tree works for subproject elements: do the same, just use
  git-diff-tree. This test is somewhat redundant, I just added it for
  completeness (diff, diff-files, and diff-index are already used)

- check if git diff works for subproject elements: try to limit
  the diff for the name of a subproject in superproject:
     git diff HEAD^ HEAD -- subproject

- check if clone works: try a clone of superproject and compare
  "git ls-files -s" output in superproject and cloned repo

- removing and adding subproject: rename test. Currently implemented
  as "git-update-index --force-remove", "mv" and "git-add".

- checkout in superproject: try to checkout the initial commit

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>