alias: prepare for subsection aliases

alias: prepare for subsection aliases

Switch git_unknown_cmd_config() from skip_prefix() to
parse_config_key() for alias parsing. This properly handles the
three-level config key structure and prepares for the new
alias.*.command subsection syntax in the next commit.

This is a compatibility break: the alias configuration parser used
to be overly permissive and accepted "alias.<subsection>.<key>" as
defining an alias "<subsection>.<key>". With this change,
alias.<subsection>.<key> entries are silently ignored (unless <key>
is "command", which will be given meaning in the next commit).

This behavior was arguably a bug, since config subsections were never
intended to work this way for aliases, and aliases with dots in their
names have never been documented or intentionally supported.

Signed-off-by: Jonatan Holmgren <jonatan@jontes.page>
Signed-off-by: Junio C Hamano <gitster@pobox.com>