In commit cd377f45c9 (refs: loosen restriction on wildcard "*"
refspecs, 2015-07-22) in git.git, the restrictions on wildcard
"*" refspecs has been loosened. While wildcards were previously
only allowed if the component is a single "*", this was changed
to also accept other patterns as part of the component.

We never adapted to that change and still reject any wildcard
patterns that aren't a single "*" only. Update our tests to
reflect the upstream change and adjust our own code accordingly.

Update internal usage to use the `git_reference` names for constants.

C++ style comment ("//") are not specified by the ISO C90 standard and
thus do not conform to it. While libgit2 aims to conform to C90, we did
not enforce it until now, which is why quite a lot of these
non-conforming comments have snuck into our codebase. Do a tree-wide
conversion of all C++ style comments to the supported C style comments
to allow us enforcing strict C90 compliance in a later commit.

Since version 8.1, GCC will do some automatic bounds checking
when printing static content into a buffer with known size. The
bounds checking doesn't yet work quite right in all scenarios and
may thus lead to false positives. Fix one of these false
positives in refs::normalize by simplifying the code.