config: split repo scope to local and worktree

config: split repo scope to local and worktree

Previously when iterating through git config variables, worktree config
and local config were both considered "CONFIG_SCOPE_REPO".  This was
never a problem before as no one had needed to differentiate between the
two cases, but future functionality may care whether or not the config
options come from a worktree or from the repository's actual local
config file.  For example, the planned feature to add a '--show-scope'
to config to allow a user to see which scope listed config options come
from would confuse users if it just printed 'repo' rather than 'local'
or 'worktree' as the documentation would lead them to expect.  As well
as the additional benefit of making the implementation look more like
how the documentation describes the interface.

To accomplish this we split out what was previously considered repo
scope to be local and worktree.

The clients of 'current_config_scope()' who cared about
CONFIG_SCOPE_REPO are also modified to similarly care about
CONFIG_SCOPE_WORKTREE and CONFIG_SCOPE_LOCAL to preserve previous behavior.

Signed-off-by: Matthew Rogers <mattr94@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>