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>

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