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

GIT_BEGIN/END_DECL were missing from sys/refs.h and preventing
compilation with g++ as the symbol were mangled.

We need to know what wchar_t and MAX_PATH are. Including common.h takes
care of that for us.

We need these directories to exist so cl_git_mkfile() can create the
files we ask it to.

Path validation may be influenced by `core.protectHFS` and
`core.protectNTFS` configuration settings, thus treebuilders
can take a repository to influence their configuration.

Validate HFS ignored char ".git" paths when `core.protectHFS` is
specified.  Validate NTFS invalid ".git" paths when `core.protectNTFS`
is specified.