CHANGELOG: add missing 0.22 changes

Better document `git_merge_commits`

The release after 0.21 is 0.22. Add a new heading for the changes since
0.22.

Bump the version number to 0.22.0 and the SOVERSION to 22.

`git_merge_commits` (and thus `git_merge`) do not use the same
strategy as `git-merge-recursive` wherein they can produce an
artificial common ancestor that is the merge of all common
ancestors.  Document this accordingly.

Fixed internal push state not being cleared when calling git_remote_download()

Include git2/common.h in sys/openssl.h.

git_remote_download() must also clear the internal push state resulting from a possible earlier push operation. Otherwise calling git_remote_update_tips() will execute the push version instead of the fetch version and among other things, tags won't be updated.

Fixed git_revert() documentation

Remote-tracking branch prunning

remote: remove git_push from the public API

Instead we provide git_remote_upload() and git_remote_update_tips() in
order to have a parallel API for fetching and pushing.

Coverity fixes

Undef stat for Mingw

treebuilder: rename _create() to _new()

Added git__timer() variant for AmigaOS.

The clock_gettime() function is normally not available under
AmigaOS, hence another solution is required. We are using now
GetUpTime() that is present in current versions of this
operating system.

This function is a constructor, so let's name it like one and leave
_create() for the reference functions, which do create/write the
reference.

Case changing rename

don't treat 0x85 as whitespace

global: include sys/openssl.h for GIT_EXPORT of fn

The openssl setup function needs to be GIT_EXPORT'ed, be sure
to include the `sys/openssl.h` header so that it is appropriately
decorated as an export function.

A byte value of 0x85 is not whitespace, we were conflating that with
U+0085 (UTF8: 0xc2 0x85).  This caused us to incorrectly treat valid
multibyte characters like U+88C5 (UTF8: 0xe8 0xa3 0x85) as whitespace.

On a case-insensitive filesystem, we need to deal with case-changing
renames (eg, foo -> FOO) by removing the old and adding the new,
exactly as if we were on a case-sensitive filesystem.

Update the `checkout::tree::can_cancel_checkout_from_notify` test, now
that notifications are always sent case sensitively.

index: reuc and name entrycounts should be size_t

For the REUC and NAME entries, we use size_t internally, and we take
size_t for the get_byindex() functions, but the entrycount() functions
strangely cast to an unsigned int instead.