Gitweb - Add flocks to deal with binary data writes and reads

Gitweb - Add flocks to deal with binary data writes and reads

There was a race condition where a downloading process could have
the file that it was reading from changed or zeroed out from
underneath it.  This adds a new locking constraint, and an
intermediate write file for the binary file.

This will write the resulting file into a new file, which will than
wait for an exclusive lock to come up before it gets moved into place.

Testing so far shows this works as expected.