Update top_dir with git repo root just when cwd locate in a git repo.

if when execute command "git rev-parse --show-toplevel" in a directory
that is not a subdirectory of a git repo, CheckCallAndFilter will not
throw exception, so the top_dir may be updated with a invalid path.

BUG=633971

Signed-off-by: hanpfei <hanpfei@gmail.com>
Review-Url: https://codereview.chromium.org/2200193003

gclient_utils.py

@@ -671,7 +671,9 @@ def GetPrimarySolutionPath():
     # checkout.
     top_dir = [os.getcwd()]
     def filter_fn(line):
-      top_dir[0] = os.path.normpath(line.rstrip('\n'))
+      repo_root_path = os.path.normpath(line.rstrip('\n'))
+      if os.path.exists(repo_root_path):
+        top_dir[0] = repo_root_path
     try:
       CheckCallAndFilter(["git", "rev-parse", "--show-toplevel"],
                          print_stdout=False, filter_fn=filter_fn)