magic pathspec: futureproof shorthand form

The earlier design was to take whatever non-alnum that the short format
parser happens to support, leaving the rest as part of the pattern, so a
version of git that knows '*' magic and a version that does not would have
behaved differently when given ":*Makefile".  The former would have
applied the '*' magic to the pattern "Makefile", while the latter would
used no magic to the pattern "*Makefile".

Instead, just reserve all non-alnum ASCII letters that are neither glob
nor regexp special as potential magic signature, and when we see a magic
that is not supported, die with an error message, just like the longhand
codepath does.

With this, ":%#!*Makefile" will always mean "%#!" magic applied to the
pattern "*Makefile", no matter what version of git is used (it is a
different matter if the version of git supports all of these three magic
matching rules).

Also make ':' without anything else to mean "there is no pathspec".  This
would allow differences between "git log" and "git log ." run from the top
level of the working tree (the latter simplifies no-op commits away from
the history) to be expressed from a subdirectory by saying "git log :".

Helped-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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