refspec: short-circuit non-pattern refspecs on transform

When transforming a non-pattern refspec, we simply need to copy over the
opposite string. Move that logic up to the wrapper so we can assume a
pattern refspec in the transformation function.
refspec: support asterisks in the middle of a pattern

We used to assume a refspec would only have an asterisk in the middle of
their respective pattern. This has not been a valid assumption for some
time now with git.

Instead of assuming where the asterisk is going to be, change the logic
to treat each pattern as having two halves with a replacement bit in the
middle, where the asterisk is.
Add a CHANGELOG entry for refspecs with asterisk in the middle
Merge pull request #2465 from libgit2/cmn/refspec-start-middle

Support refspecs with the asterisk in the middle