rebase: rebasing can also be done when HEAD is detached

Attempting to rebase when the HEAD is detached and is already
up to date with upstream (so there's nothing to do), the
following message is shown

        Current branch HEAD is up to date.

which is clearly wrong as HEAD is not a branch.

Handle the special case of HEAD correctly to give a more precise
error message.

Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: sivaraam

git-rebase.sh

@@ -601,11 +601,23 @@ then
 		test -z "$switch_to" ||
 		GIT_REFLOG_ACTION="$GIT_REFLOG_ACTION: checkout $switch_to" \
 			git checkout -q "$switch_to" --
-		say "$(eval_gettext "Current branch \$branch_name is up to date.")"
+		if test "$branch_name" = "HEAD" &&
+			 ! git symbolic-ref -q HEAD
+		then
+			say "$(eval_gettext "HEAD is up to date.")"
+		else
+			say "$(eval_gettext "Current branch \$branch_name is up to date.")"
+		fi
 		finish_rebase
 		exit 0
 	else
-		say "$(eval_gettext "Current branch \$branch_name is up to date, rebase forced.")"
+		if test "$branch_name" = "HEAD" &&
+			 ! git symbolic-ref -q HEAD
+		then
+			say "$(eval_gettext "HEAD is up to date, rebase forced.")"
+		else
+			say "$(eval_gettext "Current branch \$branch_name is up to date, rebase forced.")"
+		fi
 	fi
 fi