Mercurial appears to resolve cwd-relative path first, so "glob:*.c" could be
parsed as "**/*.c" if cwd was literally "**". It wouldn't practically matter,
but isn't correct. Instead, jj's parser first splits glob into literal part
and pattern. That's mainly because we want to parse the user input texts into
type-safe objects, and (RepoPathBuf, glob::Pattern) pairs are the simplest
ones. The current parser can't handle patterns like "foo/*/.." (= "foo" ?),
and errors out. I believe this restriction is acceptable.
Unlike literal paths, the 'glob:' pattern anchors to the whole file path. I
don't think "prefix"-matching glob is useful, and making it the default would
be rather confusing.