svn try not working

### bug
buildbot try I get result:
WARNING: There is no patch to try, diff is empty.

### debug
in tryClient.py SourceStampExtractor,I add some code to print result
def _didvc(self, res, cmd):
stdout, _, __ = res
print("SVNExtractor res", res )
print("SVNExtractor cmd", cmd)

SVNExtractor res (b'', b"svn: E205000: Syntax error in revision argument 'b'1270438''\r\n", 1)
SVNExtractor cmd ['diff', "-rb'1270438'"]

### what's wrong
in SVNExtractor.parseStatus, self.baserev will be a string
def parseStatus(self, res):
for line in res.split(b"\n"):
m = re.search(br'^Status against revision:\s+(\d+)', line)
if m:
self.baserev = m.group(1)

Start in tryClient.py with SourceStampExtractor and SVNExtractor.parseStatus, then reproduce the SVN try flow that produces the shown revision argument. Inspect how the captured revision is passed into the diff command. Done means the command no longer contains the malformed b'1270438' value and the try produces the expected patch result.

Written by the indexing model from the issue text.