[Bug] git rev-parse --git-common-dir executed from a sub dir of the main worktree is wrong

I found a case where it seems that the result of `git rev-parse --git-common-dir` is incorrect. If you execute the command from within a subdirectory in the main worktree, it returns the path from the root of the worktree to the current dir + "/.git". (As a refresher, running this command from the root of the worktree returns ".git").

I wrote a quick test to demonstrate the problem:

+test_expect_success 'git-common-dir inside sub-dir' '
+   (
+ mkdir -p path/to/child &&
+ cd path/to/child &&
+ echo "$(git rev-parse --show-toplevel)/.git" >expected &&
+ git rev-parse --git-common-dir >actual &&
+ test_cmp expected actual
+ )
+'
+

I suggest that we change the result of this call to _always_ return an absolute path. I would be willing to code this change, but I didn't want to start anything that may be considered backwards-incompatible.

This seems related to [1]http://thread.gmane.org/gmane.comp.version-control.git/286038