setup: don't perform lazy initialization of repository state

setup: don't perform lazy initialization of repository state

Under some circumstances (bogus GIT_DIR value or the discovered gitdir
is '.git') 'setup_git_directory()' won't initialize key repository
state.  This leads to inconsistent state after running the setup code.
To account for this inconsistent state, lazy initialization is done once
a caller asks for the repository's gitdir or some other piece of
repository state.  This is confusing and can be error prone.

Instead let's tighten the expected outcome of 'setup_git_directory()'
and ensure that it initializes repository state in all cases that would
have been handled by lazy initialization.

This also lets us drop the requirement to have 'have_git_dir()' check if
the environment variable GIT_DIR was set as that will be handled by the
end of the setup code.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>