git submodule: Fix adding of submodules at paths with ./, .. and //

Make 'git submodule add' normalize the submodule path in the
same way as 'git ls-files' does, so that 'git submodule init' looks up
the information in .gitmodules with the same key under which 'git
submodule add' stores it.

This fixes 4 known breakages.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -166,9 +166,18 @@ cmd_add()
 	;;
 	esac
 
-	# strip trailing slashes from path
-	path=$(echo "$path" | sed -e 's|/*$||')
-
+	# normalize path:
+	# multiple //; leading ./; /./; /../; trailing /
+	path=$(printf '%s/\n' "$path" |
+		sed -e '
+			s|//*|/|g
+			s|^\(\./\)*||
+			s|/\./|/|g
+			:start
+			s|\([^/]*\)/\.\./||
+			tstart
+			s|/*$||
+		')
 	git ls-files --error-unmatch "$path" > /dev/null 2>&1 &&
 	die "'$path' already exists in the index"