revparse: Don't return a reference when asked for a git object

Fix #1722
Switch default calling convention to cdecl.
don't include ignored as rename candidates
Merge pull request #1735 from ethomson/ignored_are_not_rename_candidates

don't include ignored as rename candidates
Merge pull request #1722 from libgit2/ntk/fix/issue_1722

git_revparse_ext: should return a NULL reference  when the revparse expression doesn't lead to a reference
Merge pull request #1736 from ben/default-to-cdecl

Switch default calling convention to cdecl
Typo