pathspec: drop "lone : means no pathspec" from get_pathspec()

pathspec: drop "lone : means no pathspec" from get_pathspec()

We may want to give the pathspec subsystem such a feature, but not while
we are still using get_pathspec() that returns a stupid "char **" that
loses subtle nuances that existed in the input string.

In the meantime, the callers of get_pathspec() that want to support it
could do an equivalent before feeding their argv[] to the function
themselves quite easily.

Signed-off-by: Junio C Hamano <redacted>