git-apply: notice "diff --git" patch again

git-apply: notice "diff --git" patch again

Earlier one that tried to be too consistent with GNU patch by
not stripping the leading path when we _know_ we are in a
subdirectory and the patch is relative to the toplevel was a
mistake.  This fixes it.

 - No change to behaviour when it is run from the toplevel of
   the repository.

 - When run from a subdirectory to apply a git-generated patch,
   it uses the right -p<n> value automatically, with or without
   --index nor --cached option.

 - When run from a subdirectory to apply a randomly generated
   patch, it wants the right -p<n> value to be given by the
   user.

The second one is a pure improvement to correct inconsistency
between --index and non --index case, compared with 1.5.0.  The
third point could be further improved to guess what the right
value for -p<n> should be by looking at the patch, but should be
a topic of a separate patch.

Signed-off-by: Junio C Hamano <junkio@cox.net>