pathspec: avoid the need of "--" when wildcard is used

pathspec: avoid the need of "--" when wildcard is used

When "--" is lacking from the command line and a command can take
both revs and paths, the idea is if an argument can be seen as both
an extended SHA-1 and a path, then "--" is required or git refuses
to continue. It's currently implemented as:

 (1) if an argument is rev, then it must not exist in worktree

 (2) else, it must exist in worktree

 (3) else, "--" is required.

These rules work for literal paths, but when non-literal pathspec is
involved, it almost always requires the user to add "--" because it
fails (2) and (1) is really rarely met (take "*.c" for example, (1)
is met if there is a ref named "*.c").

This patch modifies the rules a bit by considering any valid (*)
wildcard pathspec "exist in worktree". The rules become:

 (1) if an arg is a rev, then it must either exist in worktree or
     not be a valid wildcard pathspec.

 (2) else, it either exists in worktree or is a wildcard pathspec

 (3) else, "--" is required.

With the new rules, "--" is not needed most of the time when
wildcard pathspec is involved.

Signed-off-by: Junio C Hamano <gitster@pobox.com>