Log Message:
Changes 2.5.3:
 * The experimental untracked-cache feature were buggy when paths with
   a few levels of subdirectories are involved.

 * Recent versions of scripted "git am" has a performance regression
   in "git am --skip" codepath, which no longer exists in the
   built-in version on the 'master' front.  Fix the regression in
   the last scripted version that appear in 2.5.x maintenance track
   and older.

Also contains typofixes, documentation updates and trivial code
clean-ups.