R1:f37832aed753

Fix loop in svnserve workflow for large binaries

Summary: If you push a large binary and the data crosses multiple data frames, we can end up in a loop in the parser.

Test Plan:
After this change, I was able to push a 95MB binary in 7s, which seems reasonable:

>>> orbital ~/repos/INIS $ svn st
A large2.bin
>>> orbital ~/repos/INIS $ ls -alh
total 390648
drwxr-xr-x 6 epriestley admin 204B Dec 18 17:14 .
drwxr-xr-x 98 epriestley admin 3.3K Dec 16 11:19 ..
drwxr-xr-x 7…