scripts: objdiff: get the path to .tmp_objdiff more simply

This commit is a minor refactoring.

Temporary files for objdiff are stored in .tmp_objdiff directory
which is located at the top directory.

To get the path to this directory,

  SRCTREE=`git rev-parse --show-toplevel`
  TMPD=$SRCTREE/.tmp_objdiff

seems easier to understand than

  GIT_DIR=`git rev-parse --git-dir`
  TMPD=${GIT_DIR%git}tmp_objdiff

Besides, it is not always necessary to create .tmp_objdiff dicrectory.
It should be created only for "record" command.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Michal Marek <mmarek@suse.cz>