contrib: add sshfs-benchmark.bash

Let's get some reproducible numbers for
https://github.com/rfjakob/gocryptfs/issues/481
and
https://github.com/rfjakob/gocryptfs/issues/410

Example run:

$ ./sshfs-benchmark.bash nuetzlich.net
working directory: /tmp/sshfs-benchmark.bash.vu4
sshfs mounted: nuetzlich.net:/tmp -> sshfs.mnt
gocryptfs mounted: sshfs.mnt/sshfs-benchmark.bash.KM9/gocryptfs.crypt -> gocryptfs.mnt

                         sshfs  gocryptfs-on-sshfs
git init                  1.68               11.23
rsync                     6.07               20.35

contrib/sshfs-benchmark.bash

@@ -0,0 +1,53 @@
+#!/bin/bash
+
+set -eu
+
+function cleanup {
+	fusermount -u -z gocryptfs.mnt || true
+	rm -Rf "$SSHFS_TMP"
+	fusermount -u -z sshfs.mnt
+	cd /
+	rm -Rf "$LOCAL_TMP"
+}
+
+function prepare_mounts {
+	LOCAL_TMP=$(mktemp -d -t "$MYNAME.XXX")
+	cd $LOCAL_TMP
+	echo "working directory: $PWD"
+	mkdir sshfs.mnt gocryptfs.mnt
+	sshfs $HOST:/tmp sshfs.mnt
+	echo "sshfs mounted: $HOST:/tmp -> sshfs.mnt"
+	trap cleanup EXIT
+	SSHFS_TMP=$(mktemp -d "sshfs.mnt/$MYNAME.XXX")
+	mkdir $SSHFS_TMP/gocryptfs.crypt
+	gocryptfs -q -init -extpass "echo test" -scryptn=10 $SSHFS_TMP/gocryptfs.crypt
+	gocryptfs -q -extpass "echo test" $SSHFS_TMP/gocryptfs.crypt gocryptfs.mnt
+	echo "gocryptfs mounted: $SSHFS_TMP/gocryptfs.crypt -> gocryptfs.mnt"
+}
+
+function etime {
+	T=$(/usr/bin/time -f %e -o /dev/stdout "$@")
+	printf %20.2f "$T"
+}
+
+MYNAME=$(basename "$0")
+HOST=$1
+
+prepare_mounts
+
+# Make the bash builtin "time" print out only the elapsed wall clock
+# seconds
+TIMEFORMAT=%R
+
+echo
+echo "$MYNAME:    sshfs  gocryptfs-on-sshfs"
+echo -n "git init  "
+etime git init -q sshfs.mnt/git1
+etime git init -q gocryptfs.mnt/git1
+echo
+
+git init -q git2
+echo -n "rsync     "
+etime rsync -a --no-group git2 sshfs.mnt
+etime rsync -a --no-group git2 gocryptfs.mnt
+echo