Use absolute path when looking for git repo

When pip is installing from github, __file__ is a relative path, and by
skipping to the project root we actually created an empty string. By
making sure we work with absolute path this can be prevented.

pdc/__init__.py

@@ -9,7 +9,7 @@
 VERSION = "v0.1.0-alpha.2"
 
 old_cwd = os.getcwd()
-os.chdir(os.path.dirname(os.path.dirname(__file__)))
+os.chdir(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 # NOTE(xchu): use `git describe` when under git repository.
 if os.system('git rev-parse 2> /dev/null > /dev/null') == 0:
     pipe = subprocess.Popen("git describe",