Fix a crash when using Subvertpy 0.11.

Review Request #11725 — Created July 15, 2021 and submitted — Latest diff uploaded


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Subvertpy 0.11 introduced some new logic in client.propget() that
normalizes a path to an absolute path. This in turn triggers an eventual
assertion failure in libsvn, which checks to make sure a full repository
URL wasn't passed in (somewhere). This was not a problem in Subvertpy
0.10 (which also didn't have this path normalization).

The problem applies only to propget(), and thankfully does not apply
to proplist(). To work around this crash, we're now calling
proplist() and filtering results from that. This method avoids the
path normalization that broke us, and provides the same results.

The Subvertpy bug is tracked at:

Unit tests pass on Python 2 and 3, against Subverpty 0.10 and 0.11.