patchbomb: Defer the import of readline.
If imported at the top of the module, the import appears to succeed,
but raw_input doesn't acquire magic editing fu. I suspect this has
something to do with the newish demandimport code, because the prior
code worked with demandload.
patchbomb: Don't prompt for headers until sure we have revs to export.
The prior behaviour was to always prompt for headers, and only then
bomb out if there were actually no revs to send.
Also check the index file size when deciding whether to reload a revlog.
This should fix some spurious failures from test-clone-pull-corruption
(
issue552).