submodule: port submodule subcommand 'foreach' from shell to C

This aims to make git-submodule foreach a builtin. 'foreach' is ported to
the submodule--helper, and submodule--helper is called from
git-submodule.sh.

Helped-by: Brandon Williams <bmwill@google.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Prathamesh Chavan <pc44800@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -323,44 +323,7 @@ cmd_foreach()
 		shift
 	done
 
-	toplevel=$(pwd)
-
-	# dup stdin so that it can be restored when running the external
-	# command in the subshell (and a recursive call to this function)
-	exec 3<&0
-
-	{
-		git submodule--helper list --prefix "$wt_prefix" ||
-		echo "#unmatched" $?
-	} |
-	while read -r mode sha1 stage sm_path
-	do
-		die_if_unmatched "$mode" "$sha1"
-		if test -e "$sm_path"/.git
-		then
-			displaypath=$(git submodule--helper relative-path "$prefix$sm_path" "$wt_prefix")
-			say "$(eval_gettext "Entering '\$displaypath'")"
-			name=$(git submodule--helper name "$sm_path")
-			(
-				prefix="$prefix$sm_path/"
-				sanitize_submodule_env
-				cd "$sm_path" &&
-				# we make $path available to scripts ...
-				path=$sm_path &&
-				if test $# -eq 1
-				then
-					eval "$1"
-				else
-					"$@"
-				fi &&
-				if test -n "$recursive"
-				then
-					cmd_foreach "--recursive" "$@"
-				fi
-			) <&3 3<&- ||
-			die "$(eval_gettext "Stopping at '\$displaypath'; script returned non-zero status.")"
-		fi
-	done
+	git ${wt_prefix:+-C "$wt_prefix"} ${prefix:+--super-prefix "$prefix"} submodule--helper foreach ${GIT_QUIET:+--quiet} ${recursive:+--recursive} "$@"
 }
 
 #