Rename ".dotest/" to ".git/rebase" and ".dotest-merge" to "rebase-merge"

Since the files generated and used during a rebase are never to be
tracked, they should live in $GIT_DIR.  While at it, avoid the rather
meaningless term "dotest" to "rebase", and unhide ".dotest-merge".

This was wished for on the mailing list, but so far unimplemented.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -119,7 +119,7 @@ It does not apply to blobs recorded in its index."
 }
 
 prec=4
-dotest=".dotest"
+dotest="$GIT_DIR/rebase"
 sign= utf8=t keep= skip= interactive= resolved= binary= rebasing=
 resolvemsg= resume=
 git_apply_opt=
@@ -195,7 +195,7 @@ then
 		false
 		;;
 	esac ||
-	die "previous dotest directory $dotest still exists but mbox given."
+	die "previous rebase directory $dotest still exists but mbox given."
 	resume=yes
 else
 	# Make sure we are not given --skip nor --resolved
@@ -325,7 +325,7 @@ do
 			<"$dotest"/info >/dev/null &&
 			go_next && continue
 
-		test -s $dotest/patch || {
+		test -s "$dotest/patch" || {
 			echo "Patch is empty.  Was it split wrong?"
 			stop_here $this
 		}