config: preserve scope in do_git_config_sequence

config: preserve scope in do_git_config_sequence

do_git_config_sequence operated under the assumption that it was correct
to set current_parsing_scope to CONFIG_SCOPE_UNKNOWN as part of the
cleanup it does after it finishes execution.  This is incorrect, as it
blows away the current_parsing_scope if do_git_config_sequence is called
recursively.  As such situations are rare (git config running with the
'--blob' option is one example) this has yet to cause a problem, but the
upcoming '--show-scope' option will experience issues in that case, lets
teach do_git_config_sequence to preserve the current_parsing_scope from
before it started execution.

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