Guess source tree root from common file prefix.

* When processing a downloaded build.log file,
  find_android_root is very slow because the
  android source tree is not found.
  Change it to look for warn.py only up to 100 times,
  and then set android_root to the longest common path prefix.

Bug: 141524232
Test: warn.py --url=http://cs/android --separator='?l=' build.log > warnings.html
Change-Id: I8cc2e208d439267313aaa6cdbc00f40320409c63