run-command: add pre-exec callback

This is a function provided by the caller which is called
_after_ the process is forked, but before the spawned
program is executed. On platforms (like mingw) where
subprocesses are forked and executed in a single call, the
preexec callback is simply ignored.

This will be used in the following patch to do some setup
for 'less' that must happen in the forked child.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.h

@@ -42,6 +42,7 @@ struct child_process {
 	unsigned no_stderr:1;
 	unsigned git_cmd:1; /* if this is to be git sub-command */
 	unsigned stdout_to_stderr:1;
+	void (*preexec_cb)(void);
 };
 
 int start_command(struct child_process *);