Merge branch 'jk/common-main' into maint

There are certain house-keeping tasks that need to be performed at
the very beginning of any Git program, and programs that are not
built-in commands had to do them exactly the same way as "git"
potty does.  It was easy to make mistakes in one-off standalone
programs (like test helpers).  A common "main()" function that
calls cmd_main() of individual program has been introduced to
make it harder to make mistakes.

* jk/common-main:
  mingw: declare main()'s argv as const
  common-main: call git_setup_gettext()
  common-main: call restore_sigpipe_to_default()
  common-main: call sanitize_stdfds()
  common-main: call git_extract_argv0_path()
  add an extra level of indirection to main()

compat/mingw.h

@@ -538,7 +538,7 @@ extern CRITICAL_SECTION pinfo_cs;
 void mingw_startup(void);
 #define main(c,v) dummy_decl_mingw_main(void); \
 static int mingw_main(c,v); \
-int main(int argc, char **argv) \
+int main(int argc, const char **argv) \
 { \
 	mingw_startup(); \
 	return mingw_main(__argc, (void *)__argv); \