fix to make setup.py get_version more robust

Now it only considers a git repo in which setup.py lives at the top level.
This fixed the problem where running setup.py with a relative path from another
directory which happened to be a git repo, or installing the sdist from inside
some other git repo, would make get_version report the version of the *other*
repo.  This was initially encountered in the readthedocs.org build environment,
where the builds seem to be run within the git repo of the RTD source itself.

setup.py

@@ -71,6 +71,16 @@ class GitError(Exception):
 
 def get_version():
     """Get a nice version number from git-describe"""
+
+    # ensure that we are working in a pysb git repo
+    setup_path = os.path.abspath(os.path.dirname(__file__))
+    print setup_path
+    if not os.path.exists(os.path.join(setup_path, '.git')):
+        raise Exception("setup.py is not in the root of a git repository; "
+                        "aborting")
+    os.chdir(setup_path)
+
+    # run git describe
     gitcmd = ['git', 'describe', '--always', '--abbrev=4']
     try:
         gitproc = subprocess.Popen(gitcmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)