Bitmasks for on-disk fields of git_index_entry's flags_extended

In memory, the flags_extended fields are divided into two parts: the fields that are read from and written to disk, and other fields that in-memory only and used by libgit2. Only the flags in GIT_IDXENTRY_EXTENDED_FLAGS will get saved on-disk.

Thee first three bitmasks match the three fields in the git_index_entry flags_extended value that belong on disk. You can use them to interpret the data in the flags_extended.

The rest of the bitmasks match the other fields in the git_index_entry flags_extended value that are only used in-memory by libgit2. You can use them to interpret the data in the flags_extended.