The "test-xfer" command:

This command is used for debugging the server. There is a single argument which is the uncompressed content of an "xfer" message from client to server. This command interprets that message as if had been received by the server.

On the client side, run:

fossil push http://bogus/ --httptrace

Or a similar command to provide the output. The content of the message will appear on standard output. Capture this message into a file named (for example) out.txt. Then run the server in gdb:

gdb fossil
r test-xfer out.txt