Instead of copying over the data into the individual entries, point to
the originals, which are already in a format we can use.

win32: free thread-local data on thread exit

MinGW builds should optionally create DLLs without "lib" prefix

Custom merge drivers and proper gitattributes `merge` handling

CONTRIBUTING: document the optional tests

Since the `apply` callback can defer, the `check` callback is not
necessary.  Removing the `check` callback further makes the `payload`
unnecessary along with the `cleanup` callback.

Ensure that setting the merge attribute forces the built-in default
`text` driver and does *not* honor the `merge.default` configuration
option.  Further ensure that unsetting the merge attribute forces
a conflict (the `binary` driver).

Allow merge users to configure a custom default merge driver via
`git_merge_options`.  Similarly, honor the `merge.default` configuration
option.

When a `check` or `apply` callback function returns `GIT_EMERGECONFLICT`
stop and product a conflict.

When a `check` or `apply` callback function returns `GIT_PASSTHROUGH`,
move on to the default merge driver.

Consumers can now register custom merged drivers with
`git_merge_driver_register`.  This allows consumers to support the
merge drivers, as configured in `.gitattributes`.  Consumers will be
asked to perform the file-level merge when a custom driver is
configured.

commit: add function to attach a signature to a commit

Test memleaks

CMake: do not overwrite but only append to CMAKE_C_FLAGS_DEBUG 

commit: fix extraction of single-line signatures

The function to extract signatures suffers from a similar bug to the
header field finding one by having an unecessary line feed check as a
break condition of its loop.

Fix that and add a test for this single-line signature situation.

win32: choose the page size as our value for the page size

While often similar, these are not the same on Windows. We want to use the page
size on Windows for the pools, but for mmap we need to use the allocation
granularity as the alignment.

On the other platforms these values remain the same.

This is especially useful in combination with MinGW to yield the
Windows-compliant DLL name "git2.dll" instead of "libgit2.dll"

This is useful to force "smart" IDEs (like CLIon) to use debug
flag -g even it may have decided that "-D_DEBUG" (which is
already present) is sufficient.

In combination with the function which creates a commit into a buffer,
this allows us to more easily create signed commits.

Coverity fixes round 7

Coverity fixes round 6

Remove CI support for mingw32

Setup better defaults for OpenSSL ciphers

This ensures that when using OpenSSL a safe default set of ciphers
is selected. This is done so that the client communicates securely
and we don't accidentally enable unsafe ciphers like RC4, or even
worse some old export ciphers.

Implements the first part of https://github.com/libgit2/libgit2/issues/3682

The tests have never run successfully and we do have successful builds
of mingw-w64, so remove these CI builds which do not add value.

Don't include inttypes if compiling for Mac/iOS