Merge branch 'js/async-thread'

* js/async-thread:
  fast-import: die_nicely() back to vsnprintf (reverts part of ebaa79f)
  Enable threaded async procedures whenever pthreads is available
  Dying in an async procedure should only exit the thread, not the process.
  Reimplement async procedures using pthreads
  Windows: more pthreads functions
  Fix signature of fcntl() compatibility dummy
  Make report() from usage.c public as vreportf() and use it.
  Modernize t5530-upload-pack-error.

Conflicts:
	http-backend.c

Documentation/technical/api-run-command.txt

@@ -231,8 +231,9 @@ The function pointer in .proc has the following signature:
 
 
 There are serious restrictions on what the asynchronous function can do
-because this facility is implemented by a pipe to a forked process on
-UNIX, but by a thread in the same address space on Windows:
+because this facility is implemented by a thread in the same address
+space on most platforms (when pthreads is available), but by a pipe to
+a forked process otherwise:
 
 . It cannot change the program's state (global variables, environment,
   etc.) in a way that the caller notices; in other words, .in and .out