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.

cmake: include FindPkgConfig for windows

Apparently FindPkgConfig is not included by default on VS builds,
only Unix and Unix-like (mingw) builds.

Find libssh2 via pkg-config

Make OpenSSL locking warnings more severe

Mention the init function in the README

Show progress output on fetch for the local transport

More Windows Path checking

Fix public header on sys/refs.h