help.c: fix autocorrect in work tree for bare repository

help.c: fix autocorrect in work tree for bare repository

Currently, auto correction doesn't work reliably for commands which must
run in a work tree (e.g. `git status`) in Git work trees which are
created from a bare repository.

As far as I'm able to determine, this has been broken since commit
659fef199f (help: use early config when autocorrecting aliases,
2017-06-14), where the call to `git_config()` in `help_unknown_cmd()`
was replaced with a call to `read_early_config()`. From what I can tell,
the actual cause for the unexpected error is that we call
`git_default_config()` in the `git_unknown_cmd_config` callback instead
of simply returning `0` for config entries which we aren't interested
in.

Calling `git_default_config()` in this callback to `read_early_config()`
seems like a bad idea since those calls will initialize a bunch of state
in `environment.c` (among other things `is_bare_repository_cfg`) before
we've properly detected that we're running in a work tree.

All other callbacks provided to `read_early_config()` appear to only
extract their configurations while simply returning `0` for all other
config keys.

This commit changes the `git_unknown_cmd_config` callback to not call
`git_default_config()`. Instead we also simply return `0` for config
keys which we're not interested in.

Additionally the commit adds a new test case covering `help.autocorrect`
in a work tree created from a bare clone.

Signed-off-by: Simon Gerber <gesimu@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>