Use 'git rev-parse --git-dir' instead of assuming '.git' in top dir

This issue relates to files where we assume that mesos's '.git' folder is always located in the top level directory of the mesos tree.

For example, bootstrap makes this assumption. Specifically, it attempts to install pre-commit and post-rewrite hooks into the hardcoded .git/hooks directory. However, it is not always true that the .git folder is located here.

Most notably, it is not true if the mesos tree is included as a submodule inside another project. When included as a submodule, .git is no longer a directory, but rather a file whose text contains a pointer back to the actual location of the .git folder inside the containing project. To get at this directory, we need to run 'git rev-parse --git-dir' instead of simply assuming that the local .git is the proper directory.