Merge branch 'bw/forking-and-threading'

The "run-command" API implementation has been made more robust
against dead-locking in a threaded environment.

* bw/forking-and-threading:
  usage.c: drop set_error_handle()
  run-command: restrict PATH search to executable files
  run-command: expose is_executable function
  run-command: block signals between fork and execve
  run-command: add note about forking and threading
  run-command: handle dup2 and close errors in child
  run-command: eliminate calls to error handling functions in child
  run-command: don't die in child when duping /dev/null
  run-command: prepare child environment before forking
  string-list: add string_list_remove function
  run-command: use the async-signal-safe execv instead of execvp
  run-command: prepare command before forking
  t0061: run_command executes scripts without a #! line
  t5550: use write_script to generate post-update hook

git-compat-util.h

@@ -450,7 +450,6 @@ extern void (*get_error_routine(void))(const char *err, va_list params);
 extern void set_warn_routine(void (*routine)(const char *warn, va_list params));
 extern void (*get_warn_routine(void))(const char *warn, va_list params);
 extern void set_die_is_recursing_routine(int (*routine)(void));
-extern void set_error_handle(FILE *);
 
 extern int starts_with(const char *str, const char *prefix);