[RGC/PATCHv2] grep: submodule support

[Chris Packham <judge.packham@gmail.com>]

Here is my updated grep support for submodules series. The old series
can be seen here [1].

[1] http://article.gmane.org/gmane.comp.version-control.git/157599

This the actual implementation is far from complete but the first 3
patches should be OK on their own. Since the last series I've rebased to
master on git.git so hopefully more people can try it out if they want
to. I'll be travelling/moving for the next week or so I may be a little 
unresponsive.

The changes since last time are:

* use environment variables to pass some information to the sub-process
* a lot more options supported by create_sub_grep_argv, including
  rebuilding the pattern list.
* figure out the correct prefix based on the cwd & worktree (thanks Duy).


Chris Packham (4):
      grep: output the path from cwd to worktree
      grep_cache: check pathspec first
      add test for git grep --recursive
      grep: add support for grepping in submodules

Nguyễn Thái Ngọc Duy (1):
      worktree: provide better prefix to go back to original cwd