Test helper remote_file_equals()

It executes a diff between a local and a remote file. Makes a couple of
tests more readable.
Shortening dest file path creation

Using this substring syntax: DEST_FILE=${SRC_FILE#$SYNCROOT}
Merge branch 'develop' of github.com:git-ftp/git-ftp into develop

Conflicts:
	git-ftp
	tests/git-ftp-test.sh