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.

This introduces the functionality of submodule update in
'git_submodule_do_update'. The existing 'git_submodule_update' function is
renamed to 'git_submodule_update_strategy'. The 'git_submodule_update'
function now refers to functionality similar to `git submodule update`,
while `git_submodule_update_strategy` is used to get the configured value
of submodule.<name>.update.

Submodule init should handle relative paths in .gitmodules files
and resolve these urls when updating the git config file.

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