cpplint: make it possible to run outside git repo

cpplint uses the top-level .git directory to determine what the root is
for #include guards.  If it doesn't find a .git directory, it walks up
all the way to the system root and subsequently complains that guards
must be written as HOME_USER_SRC_NODE_SRC_FILENAME_H_.

This commit replaces the .git-based path munging with a fixed root path
relative to the location of the cpplint script, making it possible to
successfully run `make test` from an extracted tarball.

Fixes: https://github.com/nodejs/node/issues/2693
PR-URL: https://github.com/nodejs/node/pull/2710
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

tools/cpplint.py

@@ -695,38 +695,11 @@ class FileInfo:
     locations won't see bogus errors.
     """
     fullname = self.FullName()
-
-    if os.path.exists(fullname):
-      project_dir = os.path.dirname(fullname)
-
-      if os.path.exists(os.path.join(project_dir, ".svn")):
-        # If there's a .svn file in the current directory, we recursively look
-        # up the directory tree for the top of the SVN checkout
-        root_dir = project_dir
-        one_up_dir = os.path.dirname(root_dir)
-        while os.path.exists(os.path.join(one_up_dir, ".svn")):
-          root_dir = os.path.dirname(root_dir)
-          one_up_dir = os.path.dirname(one_up_dir)
-
-        prefix = os.path.commonprefix([root_dir, project_dir])
+    # XXX(bnoordhuis) Expects that cpplint.py lives in the tools/ directory.
+    toplevel = os.path.abspath(os.path.join(os.path.dirname(__file__), '..'))
+    prefix = os.path.commonprefix([fullname, toplevel])
     return fullname[len(prefix) + 1:]
 
-      # Not SVN? Try to find a git or hg top level directory by searching up
-      # from the current path.
-      root_dir = os.path.dirname(fullname)
-      while (root_dir != os.path.dirname(root_dir) and
-             not os.path.exists(os.path.join(root_dir, ".git")) and
-             not os.path.exists(os.path.join(root_dir, ".hg"))):
-        root_dir = os.path.dirname(root_dir)
-
-      if (os.path.exists(os.path.join(root_dir, ".git")) or
-          os.path.exists(os.path.join(root_dir, ".hg"))):
-        prefix = os.path.commonprefix([root_dir, project_dir])
-        return fullname[len(prefix) + 1:]
-
-    # Don't know what to do; header guard warnings may be wrong...
-    return fullname
-
   def Split(self):
     """Splits the file into the directory, basename, and extension.