Add a helper script to clean up .gitignore files

Yours truly uses this script to clean up Fiji's .gitignore file.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

validate-gitignore.sh

@@ -0,0 +1,63 @@
+#!/bin/sh
+
+# This script can be used to identify unnecessary entries in the .gitignore
+# file.
+#
+# Three modes are available:
+#
+# --validate
+#	prints out unnecessary entries
+# --clean
+#	prints out the contents of .gitignore, skipping unnecessary entries
+# --fix
+#	overwrites .gitignore with the output of --clean
+
+die () {
+	echo "$*" >&2
+	exit 1
+}
+
+while test $# -gt 0
+do
+	case "$1" in
+	--fix|--validate|--clean) mode=${1#--};;
+	*) die "Unknown command: $1";;
+	esac
+	shift
+done
+
+test -f .gitignore ||
+die "No .gitignore file?"
+
+handle_line () {
+	case "$1" in
+	''|\#*)
+		printf '%s\n' "$1"
+		continue
+		;;
+	/*)
+		if eval ls -d ."$1" > /dev/null 2>&1
+		then
+			printf "%s\n" "$1"
+		elif test "validate" = "$mode"
+		then
+			echo "Unnecessary: $1" 1>&2
+		fi
+		;;
+	esac
+}
+
+cleaned="$(cat .gitignore |
+	while read line 
+	do
+		handle_line "$line"
+	done)"
+
+case "$mode" in
+fix)
+	printf "%s" "$cleaned" > .gitignore
+	;;
+clean)
+	printf "%s" "$cleaned"
+	;;
+esac