Merge branch 'bw/forking-and-threading' into maint

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

Author: gitster

git-compat-util.h

@@ -445,7 +445,6 @@ 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);
 

help.c

@@ -1,6 +1,7 @@
 #include "cache.h"
 #include "builtin.h"
 #include "exec_cmd.h"
+#include "run-command.h"
 #include "levenshtein.h"
 #include "help.h"
 #include "common-cmds.h"
@@ -96,48 +97,6 @@ static void pretty_print_cmdnames(struct cmdnames *cmds, unsigned int colopts)
 	string_list_clear(&list, 0);
 }
 
-static int is_executable(const char *name)
-{
-	struct stat st;
-
-	if (stat(name, &st) || /* stat, not lstat */
-	    !S_ISREG(st.st_mode))
-		return 0;
-
-#if defined(GIT_WINDOWS_NATIVE)
-	/*
-	 * On Windows there is no executable bit. The file extension
-	 * indicates whether it can be run as an executable, and Git
-	 * has special-handling to detect scripts and launch them
-	 * through the indicated script interpreter. We test for the
-	 * file extension first because virus scanners may make
-	 * it quite expensive to open many files.
-	 */
-	if (ends_with(name, ".exe"))
-		return S_IXUSR;
-
-{
-	/*
-	 * Now that we know it does not have an executable extension,
-	 * peek into the file instead.
-	 */
-	char buf[3] = { 0 };
-	int n;
-	int fd = open(name, O_RDONLY);
-	st.st_mode &= ~S_IXUSR;
-	if (fd >= 0) {
-		n = read(fd, buf, 2);
-		if (n == 2)
-			/* look for a she-bang */
-			if (!strcmp(buf, "#!"))
-				st.st_mode |= S_IXUSR;
-		close(fd);
-	}
-}
-#endif
-	return st.st_mode & S_IXUSR;
-}
-
 static void list_commands_in_dir(struct cmdnames *cmds,
 					 const char *path,
 					 const char *prefix)