We _dispose_ the contents of objects; we _free_ objects (and their
contents).  Update `git_strarray_free` to be `git_strarray_dispose`.
`git_strarray_free` remains as a deprecated proxy function.

Right now, the function `git_repository_foreach_head` will always
iterate over all HEADs of the main repository and its worktrees. In some
cases, it might be required to skip either of those, though. Add a flag
in preparation for the following commit that enables this behaviour.

The documentation states that git_worktree_unlock returns 0 on success,
and 1 on success if the worktree wasn't locked. Turns out we were
returning 0 in any of those cases.

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.