Doc fixes

The description of what the function does hasn't been true for quite a
while. Change it to reflect the way it currently works.

While here, remove an even older comment about missing features that
have been implemented.

clang's docparser highlighted these.

Invalid refs on disk cause revwalk globbing to fail

The new tests don't always want to use the same fixture data as
the old ones so this makes it configurable on a per-test basis.

Instead of using lots of strdup calls, this adds a memory pool to
the loose refs iteration code and uses it for keeping track of the
loose refs array.  Memory usage could probably be reduced even
further by eliminating the vector and just scanning by adding the
strlen of each ref, but that would be a more intrusive changes.

This also updates the error handling to be more thorough about
checking for failed allocations, etc.

The git_reference_next API silently skips invalid references when
scanning the loose refs.  The git_reference_next_name API should
skip the same ones even though it isn't creating the reference
object.

This adds a test with a an invalid loose reference and makes sure
that both APIs skip the same entries and generate the same results.

Clarify when to use github issues

git_buf_text_gather_stats doesn't work for multi-byte characters

Suggest that github issues are to be used for bug reports, while questions about usage should be directed to StackOverflow.

Refresh readme and contributing guidance

Updating the contributing guidance to explain a bit more about how we use
PR's

Updated the methods of getting involved with the project and asking
questions.

Switch default calling convention to cdecl

git_revparse_ext: should return a NULL reference  when the revparse expression doesn't lead to a reference

don't include ignored as rename candidates

Small grammar fix in docs

Small fixes

Add `git_remote_owner`.

This reverts commit a91e4d6b.

Fixes return type documentation