keywords: safely encode symlink keywords

symlink(2) is a very dumb syscall, and allows you to put any damn value
you please inside a symlink. In addition, spaces are valid path
characters which causes issues with keyword parsing. So use Vis() and
Unvis() to safely store an encoded version of the path.

This also adds a cli test for this behaviour.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

test/cli/0002.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+set -e
+
+name=$(basename $0)
+root=$1
+gomtree=$(readlink -f ${root}/gomtree)
+t=$(mktemp -d /tmp/go-mtree.XXXXXX)
+
+echo "[${name}] Running in ${t}"
+# This test is for basic running check of manifest, and check against tar and file system
+#
+
+pushd ${root}
+
+# Create a symlink with spaces in the entries.
+mkdir ${t}/root
+ln -s "this is a dummy symlink" ${t}/root/link
+
+# Create manifest and check it against the same symlink.
+${gomtree} -K link,sha256digest -c -p ${t}/root > ${t}/root.mtree
+${gomtree} -K link,sha256digest -f ${t}/root.mtree -p ${t}/root
+
+popd
+rm -rf ${t}