git: remove an early return from save_env_before_alias()

When help.autocorrect is in effect, an attempt to auto-execute an uniquely corrected result of a misspelt alias will result in an irrelevant error message. The codepath that causes this calls save_env_before_alias() and restore_env() in handle_alias(), and that happens twice. A global variable orig_cwd is allocated to hold the return value of getcwd() in save_env_before_alias(), which is then used in restore_env() to go back to that directory and finally free(3)'d there. However, save_env_before_alias() is not prepared to be called twice. It returns early when it knows it has already been called, leaving orig_cwd undefined, which is then checked in the second call to restore_env(), and by that time, the memory that used to hold the contents of orig_cwd is either freed or reused to hold something else, and this is fed to chdir(2), causing it to fail. Even if it did not fail (i.e. reading of the already free'd piece of memory yielded a directory path that we can chdir(2) to), it then gets free(3)'d. Fix this by making sure save_env() does do the saving when called. While at it, add a minimal test for help.autocorrect facility. Signed-off-by: Junio C Hamano <gitster@pobox.com>