A stream used by the smart transport to read and write data
from a subtransport.
This provides a customization point in case you need to
support some other communication method.
int (*)(git_smart_subtransport_stream *, char *, size_t, size_t *)
read
int (*)(git_smart_subtransport_stream *, const char *, size_t)
write
void (*)(git_smart_subtransport_stream *)
free
