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>

scripts/objdiff

@@ -25,17 +25,15 @@
 #
 # Note: 'make mrproper' will also remove .tmp_objdiff
 
-GIT_DIR="`git rev-parse --git-dir`"
+SRCTREE=$(git rev-parse --show-toplevel 2>/dev/null)
 
-if [ -d "$GIT_DIR" ]; then
-	TMPD="${GIT_DIR%git}tmp_objdiff"
-
-	[ -d "$TMPD" ] || mkdir "$TMPD"
-else
-	echo "ERROR: git directory not found."
+if [ -z "$SRCTREE" ]; then
+	echo "ERROR: Not a git repository."
 	exit 1
 fi
 
+TMPD=$SRCTREE/.tmp_objdiff
+
 usage() {
 	echo "Usage: $0 <command> <args>"
 	echo "  record    <list of object files>"