Catch errors calling git for commit ID

Closes gh-2468

notebook/_sysinfo.py

@@ -46,11 +46,15 @@ def pkg_commit_hash(pkg_path):
     while cur_path != par_path:
         cur_path = par_path
         if p.exists(p.join(cur_path, '.git')):
-            proc = subprocess.Popen(['git', 'rev-parse', '--short', 'HEAD'],
-                                stdout=subprocess.PIPE,
-                                stderr=subprocess.PIPE,
-                                cwd=pkg_path)
-            repo_commit, _ = proc.communicate()
+            try:
+                proc = subprocess.Popen(['git', 'rev-parse', '--short', 'HEAD'],
+                                    stdout=subprocess.PIPE,
+                                    stderr=subprocess.PIPE,
+                                    cwd=pkg_path)
+                repo_commit, _ = proc.communicate()
+            except OSError:
+                repo_commit = None
+
             if repo_commit:
                 return 'repository', repo_commit.strip().decode('ascii')
             else: