Add git_index_add_entry_with_custom_stat

allows for putting entries in the index where not just
the (OID, path, filemode) are specified but also the stat info.

Add git_odb_set_lookup_flags

- supports setting git_odb_lookup_flags_t that take effect until they
are next changed. Flags have the same effect as when they are passed
directly in git_odb_exists_ext.

Add git_odb_set_lookup_flags

- supports setting git_odb_lookup_flags_t that take effect until they
are next changed. Flags have the same effect as when they are passed
directly in git_odb_exists_ext.

Add git_odb_set_lookup_flags

- supports setting git_odb_lookup_flags_t that take effect until they
are next changed. Flags have the same effect as when they are passed
directly in git_odb_exists_ext.

Add flag to git_index_write_tree -

specifically, allow for disabling the validation of every OID.

Merge pull request libgit2#4983 from libgit2/ethomson/v0_28_1

Release v0.28.1

Merge pull request libgit2#4970 from libgit2/ethomson/0_28

v0.28 rc1

Merge pull request libgit2#4970 from libgit2/ethomson/0_28

v0.28 rc1

CHANGELOG: there's changes (plural) not just one

Merge pull request libgit2#4846 from pks-t/pks/v0.27.6

Bugix release v0.27.7