filerepo: Replace confusing substr() with rtrim()

filerepo: Replace confusing substr() with rtrim()

I just looked at this code for ten minutes to understand what
sub-string is extracted here. Not only are the numbers hard to
understand: With 0 and -1 the last character is removed – that's
really all this does. Even more problematic is the fact there is
no check, hint, not even a comment explaining *which* character is
removed. If – for whatever reason – the code above returns a string
that does *not* end with a slash, the unconditional substr() call
must destroy this string.

rtrim() shows the character.

I checked and these strings are all guaranteed to use forward
slashes, never backwards (Windows) slashes.

Change-Id: I2e17fd583982920bb8a0ca73035094099e5e5d31