submodule: add get_submodule_config helper funtion

Several submodule configuration variables
(e.g. fetchRecurseSubmodules) are read from .gitmodules with local
overrides from the usual git config files.  This shell function mimics
that logic to help initialize configuration variables in
git-submodule.sh.

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -152,6 +152,32 @@ die_if_unmatched ()
 	fi
 }
 
+#
+# Print a submodule configuration setting
+#
+# $1 = submodule name
+# $2 = option name
+# $3 = default value
+#
+# Checks in the usual git-config places first (for overrides),
+# otherwise it falls back on .gitmodules.  This allows you to
+# distribute project-wide defaults in .gitmodules, while still
+# customizing individual repositories if necessary.  If the option is
+# not in .gitmodules either, print a default value.
+#
+get_submodule_config () {
+	name="$1"
+	option="$2"
+	default="$3"
+	value=$(git config submodule."$name"."$option")
+	if test -z "$value"
+	then
+		value=$(git config -f .gitmodules submodule."$name"."$option")
+	fi
+	printf '%s' "${value:-$default}"
+}
+
+
 #
 # Map submodule path to submodule name
 #