Simplify handling of setup_git_directory_gently() failure cases.

Simplify handling of setup_git_directory_gently() failure cases.

setup_git_directory_gently() expects two types of failures to
discover a git directory (e.g. .git/):

  - GIT_DIR_HIT_CEILING: could not find a git directory in any
	parent directories of the cwd.
  - GIT_DIR_HIT_MOUNT_POINT: could not find a git directory in
	any parent directories up to the mount point of the cwd.

Both cases are handled in a similar way, but there are misleading
and unimportant differences. In both cases, setup_git_directory_gently()
should:

  - Die if we are not in a git repository. Otherwise:
  - Set nongit_ok = 1, indicating that we are not in a git repository
	but this is ok.
  - Call strbuf_release() on any non-static struct strbufs that we
	allocated.

Before this change are two misleading additional behaviors:

  - GIT_DIR_HIT_CEILING: setup_nongit() changes to the cwd for no
	apparent reason. We never had the chance to change directories
	up to this point so chdir(current cwd) is pointless.
  - GIT_DIR_HIT_MOUNT_POINT: strbuf_release() frees the buffer
	of a static struct strbuf (cwd). This is unnecessary because the
	struct is static so its buffer is always reachable. This is also
	misleading because nowhere else in the function is this buffer
	released.

This change eliminates these two misleading additional behaviors and
deletes setup_nogit() because the code is clearer without it. The
result is that we can see clearly that GIT_DIR_HIT_CEILING and
GIT_DIR_HIT_MOUNT_POINT lead to the same behavior (ignoring the
different help messages).

During review, this change was amended to additionally include:

  - Neither GIT_DIR_HIT_CEILING nor GIT_DIR_HIT_MOUNT_POINT may
	return early from setup_git_directory_gently() before the
	GIT_PREFIX environment variable is reset. Change both cases to
	break instead of return. See GIT_PREFIX below for more details.

  - GIT_DIR_NONE: setup_git_directory_gently_1() never returns this
	value, but if it ever did, setup_git_directory_gently() would
	incorrectly record that it had found a repository. Explicitly
	BUG on this case because it is underspecified.

  - GIT_PREFIX: this environment variable must always match the
	value of startup_info->prefix and the prefix returned from
	setup_git_directory_gently(). Make how we handle this slightly
	more repetitive but also more clear.

  - setup_git_env() and repo_set_hash_algo(): Add comments showing
	that only GIT_DIR_EXPLICIT, GIT_DIR_DISCOVERED, and GIT_DIR_BARE
	will cause setup_git_directory_gently() to call these setup
	functions. This was obvious (but partly incorrect) before this
	change when GIT_DIR_HIT_MOUNT_POINT returned early from
	setup_git_directory_gently().

Signed-off-by: Junio C Hamano <gitster@pobox.com>