Upstream git.git has converted its codebase to use wildcard in
favor of fnmatch in commit 70a8fc999d (stop using fnmatch (either
native or compat), 2014-02-15). To keep our own regex-matching in
line with what git does, convert all trivial instances of
`fnmatch` usage to use `wildcard`, instead. Trivial usage is
defined to be use of `fnmatch` with either no flags or flags that
have a 1:1 equivalent in wildmatch (PATHNAME, IGNORECASE).

We're about to phase out our bundled fnmatch implementation as
git.git has moved to wildmatch long ago in 2014. To make it
easier to spot which files are stilll using fnmatch, remove the
implicit "fnmatch.h" include in "posix.h" and instead include it
explicitly.

When we transform a refspec with a component containing a glob, then
we simply copy over the component until the next separator from
the matching ref. E.g. if we have a ref "refs/heads/foo/bar" and
a refspec "refs/heads/*/bar:refs/remotes/origin/*/bar", we:

1. Copy over everything until hitting the glob from the <dst>
   part: "refs/remotes/origin/".
2. Strip the common prefix of ref and <src> part until the glob,
   which is "refs/heads/". This leaves us with a ref of "foo/bar".
3. Copy from the ref until the next "/" separator, resulting in
   "refs/remotes/origin/foo".
4. Copy over the remaining part of the <dst> spec, which is
   "bar": "refs/remotes/origin/foo/bar".

This worked just fine in a world where globs in refspecs were
restricted such that a globbing component may only contain a
single "*", only. But this restriction has been lifted, so that a
glob component may be nested between other characters, causing
the above algorithm to fail. Most notably the third step, where
we copy until hitting the next "/" separator, might result in a
wrong transformation. Given e.g. a ref "refs/gbranchg/head" and a
refspec "refs/g*g/head:refs/remotes/origin/*", we'd also be
copying the "g" between "branch" and "/" and end up with the
wrong transformed ref "refs/remotes/origin/branchg".

Instead of copying until the next component separator, we should
copy until we hit the pattern after the "*". So in the above
example, we'd copy until hitting the string "g/head".

Move to the `git_error` name in the internal API for error-related
functions.

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.

git_refspec_transform: Handle NULL dst

Since commit 630a67366 (refspec: add public parsing api, 2018-02-07), we
now have two functions `git_refspec_free` and `git_refspec__free`. The
difference is that the first one will free the structure itself, while
the second one will only free the structure's contents. Use our new
`dispose` naming pattern for the latter function to help avoid
confusion.

Fix typo

Fix some type issues

More fixes

Address requested changes

Add test

Fix naming

Fix condition and tests

Address requested changes

Fix typo