Make git_oid_equal a non-inline API
Clarify C compatibility policy

and a couple of other minor doc fixups.
Address PR comments

* Make GIT_INLINE an internal definition so it cannot be used in
  public headers
* Fix language in CONTRIBUTING
* Make index caps API use signed instead of unsigned values
Merge pull request #2129 from arrbee/rb/c89-or-c99

Improve C89 compatibility and clarify use of C99 features