help.autocorrect: do not run a command if the command given is junk

help.autocorrect: do not run a command if the command given is junk

If a given command is not found, then help.c tries to guess which one the
user could have meant. If help.autocorrect is 0 or unset, then a list of
suggestions is given as long as the dissimilarity between the given command
and the candidates is not excessively high. But if help.autocorrect was
non-zero (i.e., a delay after which the command is run automatically), the
latter restriction on dissimilarity was not obeyed.

In my case, this happened:

 $ git ..daab02
 WARNING: You called a Git command named '..daab02', which does not exist.
 Continuing under the assumption that you meant 'read-tree'
 in 4.0 seconds automatically...

The patch reuses the similarity limit that is also applied when the list of
suggested commands is printed.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>