alias: support non-alphanumeric names via subsection syntax

alias: support non-alphanumeric names via subsection syntax

Git alias names are limited to ASCII alphanumeric characters and
dashes because aliases are implemented as config variable names.
This prevents aliases being created in languages using characters outside that range.

Add support for arbitrary alias names by using config subsections:

    [alias "förgrena"]
        command = branch

The subsection name is matched as-is (case-sensitive byte comparison),
while the existing definition without a subsection (e.g.,
"[alias] co = checkout") remains case-insensitive for backward
compatibility. This uses existing config infrastructure since
subsections already support arbitrary bytes, and avoids introducing
Unicode normalization.

Also teach the help subsystem about the new syntax so that "git help
-a" properly lists subsection aliases and the autocorrect feature can
suggest them. Use utf8_strwidth() instead of strlen() for column
alignment so that non-ASCII alias names display correctly.

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