When PATH contains an unreadable directory, alias expansion code did
not kick in, and failed with an error that said "git-subcmd" was not
found.

By Jeff King (1) and Ramsay Jones (1)
* jk/run-command-eacces:
  run-command: treat inaccessible directories as ENOENT
  compat/mingw.[ch]: Change return type of exec functions to int