We have various macro, enumeration and structure names that were
introduced (very) early in the project and do not match our current
naming conventions.  For instance: `GIT_IDXENTRY...` flags that
correspond to a structure named `git_index_entry`.

Update these to match the current guidance.  The old macros and
enumeration names are reflected as new macros in order to support
backward compatibility (and do so without warnings for consumers).