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.c

@@ -110,6 +110,8 @@ int start_command(struct child_process *cmd)
 					unsetenv(*cmd->env);
 			}
 		}
+		if (cmd->preexec_cb)
+			cmd->preexec_cb();
 		if (cmd->git_cmd) {
 			execv_git_cmd(cmd->argv);
 		} else {