rebase: introduce and use pseudo-ref REBASE_HEAD

The new command `git rebase --show-current-patch` is useful for seeing
the commit related to the current rebase state. Some however may find
the "git show" command behind it too limiting. You may want to
increase context lines, do a diff that ignores whitespaces...

For these advanced use cases, the user can execute any command they
want with the new pseudo ref REBASE_HEAD.

This also helps show where the stopped commit is from, which is hard
to see from the previous patch which implements --show-current-patch.

Helped-by: Tim Landscheidt <tim@tim-landscheidt.de>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--merge.sh

@@ -57,6 +57,7 @@ call_merge () {
 	echo "$msgnum" >"$state_dir/msgnum"
 	cmt="$(cat "$state_dir/cmt.$msgnum")"
 	echo "$cmt" > "$state_dir/current"
+	git update-ref REBASE_HEAD "$cmt"
 	hd=$(git rev-parse --verify HEAD)
 	cmt_name=$(git symbolic-ref HEAD 2> /dev/null || echo HEAD)
 	eval GITHEAD_$cmt='"${cmt_name##refs/heads/}~$(($end - $msgnum))"'
@@ -138,13 +139,14 @@ skip)
 	return
 	;;
 show-current-patch)
-	exec git show "$(cat "$state_dir/current")" --
+	exec git show REBASE_HEAD --
 	;;
 esac
 
 mkdir -p "$state_dir"
 echo "$onto_name" > "$state_dir/onto_name"
 write_basic_state
+rm -f "$(git rev-parse --git-path REBASE_HEAD)"
 
 msgnum=0
 for cmt in $(git rev-list --reverse --no-merges "$revisions")