bug#10472: `realpath --relative-to=<path> /` outputs inconsistent trailing slash

[Mike Frysinger]

first some examples that look fine ...

these all output the same thing:
        realpath --relative-to=/usr /usr/bin
        realpath --relative-to=/usr/ /usr/bin
        realpath --relative-to=/usr /usr/bin/
        realpath --relative-to=/usr/ /usr/bin/
which is to say, they show:
        bin

as does these:
        realpath --relative-to=/usr/bin /usr
        realpath --relative-to=/usr/bin/ /usr
        realpath --relative-to=/usr/bin /usr/
        realpath --relative-to=/usr/bin/ /usr/
which is to say, they show:
        ..

as does these:
        realpath --relative-to=/ /usr
        realpath --relative-to=/ /usr/
which is to say, they show:
        ..

however, if the last argument is just the root path:
        realpath --relative-to=/usr /
        realpath --relative-to=/usr/ /
we end up with a trailing slash:
        ../

for consistency, i don't think that should be the case

(reported by Ulrich Müller via https://bugs.gentoo.org/398339)
-mike

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