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
Tidy up the methods of contacting the project

Updated the methods of getting involved with the project and asking
questions.
Update contributing guidance to explain PR flow

Updating the contributing guidance to explain a bit more about how we use
PR's
Merge pull request #1742 from martinwoodward/Refresh-Readme

Refresh readme and contributing guidance