Show warning to user when silently ignoring pushed files

[jaywink]

When sending files with command df p -f requirements.txt for example, the toolbelt will silently ignore the files and report "unchanged" if not BRANCH=master.

This caused me quite a bit of head scratching - sending files was working fine locally (master checked out) but not in Travis (PR merge to master ie on a commit). There is nothing in the help files of the toolbelt (at least cli help) indicating that BRANCH=master is a hard requirement or the client will act as if nothing happened. Even changing the file names was still giving "unchanged" which is very confusing.

Some kind of warning to the user might be nice when some kind of automatic silent ignoring is being done.

Comment which led me to the silent ignoring: #2 (comment)