alias: treat empty subsection [alias ""] as plain [alias]

When git-config stores a key of the form alias..name, it records it under an empty subsection ([alias ""]). The new subsection-aware alias lookup would see a non-NULL but zero-length subsection and fall into the subsection code path, where it required a "command" key and thus silently ignored the entry. Normalize an empty subsection to NULL before any further processing so that entries stored this way continue to work as plain case-insensitive aliases, matching the pre-subsection behaviour. Users who relied on alias..name to create an alias literally named ".name" may want to migrate to subsection syntax, which looks less confusing: [alias ".name"] command = <value> Add tests covering both the empty-subsection compatibility case and the leading-dot alias via the new syntax. Signed-off-by: Jonatan Holmgren <jonatan@jontes.page> Signed-off-by: Junio C Hamano <gitster@pobox.com>