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.

Verified that this breaks before the fix and passes afterwards.

Fix typo

Fix some type issues

More fixes

Address requested changes

Add test

Fix naming

Fix condition and tests

Address requested changes

Fix typo

Two tests in network::fetchlocal explicitly set a cleanup function to
free and remove the created sandbox repositories. This is not necessary,
though, as the cleanup function executed after each test already takes
care of cleaning up after them. Remove the code to avoid needless code
duplication.