Check project path to make sure it actully exists

Some old projects might be removed for some reason, especially
the downstream ones. It should be fine to just skip analysing them
because people have stopped working on them.

Test: run the script against an old release
Change-Id: I563905565c4c502036159fce6a386bba13ba25ea

Author: Po-Chien Hsueh

tools/repo_diff/repo_diff_trees.py

@@ -45,6 +45,11 @@ def get_projects(source_tree):
     path = project.get('path', project.get('name'))
     path = os.path.abspath(os.path.join(source_tree, path))
     name = project.get('name')
+
+    # check if project files actually exist
+    if not os.path.exists(path):
+      continue
+
     projects[name] = path
 
   return projects