Fix sed string regex escaping in module_name.

When escaping a string to be used as a sed regex, it is important
to only escape active characters.  Escaping other characters is
undefined according to POSIX, and in practice leads to issues with
extensions such as GNU sed's \+.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -47,7 +47,7 @@ get_repo_base() {
 module_name()
 {
 	# Do we have "submodule.<something>.path = $1" defined in .gitmodules file?
-	re=$(printf '%s' "$1" | sed -e 's/\([^a-zA-Z0-9_]\)/\\\1/g')
+	re=$(printf '%s' "$1" | sed -e 's/[].[^$\\*]/\\&/g')
 	name=$( GIT_CONFIG=.gitmodules \
 		git config --get-regexp '^submodule\..*\.path$' |
 		sed -n -e 's|^submodule\.\(.*\)\.path '"$re"'$|\1|p' )