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.

HFS filesystems ignore some characters like U+200C.  When these
characters are included in a path, they will be ignored for the
purposes of comparison with other paths.  Thus, if you have a ".git"
folder, a folder of ".git<U+200C>" will also match.  Protect our
".git" folder by ensuring that ".git<U+200C>" and friends do not match it.

Validate loose reference names on Win32.

Disallow:
 1. paths with trailing dot
 2. paths with trailing space
 3. paths with trailing colon
 4. paths that are 8.3 short names of .git folders ("GIT~1")
 5. paths that are reserved path names (COM1, LPT1, etc).
 6. paths with reserved DOS characters (colons, asterisks, etc)

These paths would (without \\?\ syntax) be elided to other paths - for
example, ".git." would be written as ".git".  As a result, writing these
paths literally (using \\?\ syntax) makes them hard to operate with from
the shell, Windows Explorer or other tools.  Disallow these.

When turning UTF-8 paths into UCS-2 paths for Windows, always use
the \\?\-prefixed paths.  Because this bypasses the system's
path canonicalization, handle the canonicalization functions ourselves.

We must:
 1. always use a backslash as a directory separator
 2. only use a single backslash between directories
 3. not rely on the system to translate "." and ".." in paths
 4. remove trailing backslashes, except at the drive root (C:\)

Pretend we have a git process at the other end by creating a similar
progress output when inserting objects into the packbuilder.

We do not currently generate any messages when we're counting the
objects, as might be expected from a local upload-pack. Assert that we
do call the function when working.

This fixes #2731.

It seems these were forgotten when initially splitting this up.

Our git_openssl_set_locking() would ideally not exist. Make it clearer
that we provide it as a last resort and you should prefer anything else.

Fold `git_push_unpack_ok()` into `git_push_finish()`

This should make it clearer what the return value implies.

The push cannot be successful if we sent a bad packfile. We should
return an error in that case instead of storing it elsewhere.

Introduce stackable IO streams