Mark some symbols to be deprecated #minor (libgit2#698)

This change introduces the file deprecated.go, which contains any
constants, functions, and types that are slated to be deprecated in the
next major release.

These symbols are deprecated because they refer to old spellings in
pre-1.0 libgit2. This also makes the build be done with the
`-DDEPRECATE_HARD` flag to avoid regressions.

This, together with
[gorelease](https://godoc.org/golang.org/x/exp/cmd/gorelease)[1] should
make releases safer going forward.

1: More information about how that works at
   https://go.googlesource.com/exp/+/refs/heads/master/apidiff/README.md

Relax libgit2 minor version check (libgit2#696) (libgit2#697)

The major version must still be an exact match since libgit2 uses
semantic versioning and changes to the major number indicate backwards
incompatible changes to the API.

Fixes: libgit2#695
(cherry picked from commit 1fabe95)

Relax libgit2 minor version check (libgit2#696)

The major version must still be an exact match since libgit2 uses
semantic versioning and changes to the major number indicate backwards
incompatible changes to the API.

Fixes: libgit2#695

Expose GIT_CERT_SSH_SHA256 (libgit2#690)

Newer versions of libssh2 use SHA256 fingerprints

Expose GIT_CERT_SSH_SHA256 (libgit2#690) (libgit2#694)

Newer versions of libssh2 use SHA256 fingerprints

(cherry picked from commit 7497529)

Co-authored-by: Hans Duedal <[email protected]>

Add ReferenceNormalizeName (libgit2#681)

Add ReferenceNormalizeName (libgit2#681) (libgit2#686)

(cherry picked from commit 2bd574b)

Co-authored-by: Segev Finer <[email protected]>

Add ReferenceNormalizeName (libgit2#681) (libgit2#687)

(cherry picked from commit 2bd574b)

Co-authored-by: Segev Finer <[email protected]>

Travis-ci: added support for ppc64le (libgit2#682) (libgit2#685)

Added power support for the travis.yml file with ppc64le. This is part of the Ubuntu distribution for ppc64le. This helps us simplify testing later when distributions are re-building and re-releasing.

(cherry picked from commit 2d639d8)

Co-authored-by: Devendra <[email protected]>

Add ReferenceNormalizeName (libgit2#681) (libgit2#688)

(cherry picked from commit 2bd574b)

Co-authored-by: Segev Finer <[email protected]>