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>