run-command: add note about forking and threading

run-command: add note about forking and threading

All non-Async-Signal-Safe functions (e.g. malloc and die) were removed
between 'fork' and 'exec' in start_command in order to avoid potential
deadlocking when forking while multiple threads are running.  This
deadlocking is possible when a thread (other than the one forking) has
acquired a lock and didn't get around to releasing it before the fork.
This leaves the lock in a locked state in the resulting process with no
hope of it ever being released.

Add a note describing this potential pitfall before the call to 'fork()'
so people working in this section of the code know to only use
Async-Signal-Safe functions in the child process.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>