Reading patch files

Improve the README

When showing copy information because we are duplicating contents,
for example, when performing a `diff --find-copies-harder -M100 -B100`,
then show copy from/to lines in a patch, and do not show context.
Ensure that we can also parse such patches.

Patches may have no hunks when there's no modifications (for example,
in a rename).  Handle them.

Test that we can create a diff file, then parse the results and
that the two are identical in-memory.

Cleanups

cmake: do not use -fPIC for MSYS2

Avoid declaring old-style functions without any parameters.
Functions not accepting any parameters should be declared with
`void fn(void)`. See ISO C89 $3.5.4.3.

The MSYS2 build system automatically compiles all code with position-independent
code. When we manually add the -fPIC flag to the compiler flags, MSYS2 will
loudly complain about PIC being the default and thus not required.

Fix the annoyance by stripping -fPIC in MSYS2 enviroments like it is already
done for MinGW.

Thread namespacing

Drop node-gitteh.  Replace outdated PowerShell bindings with PSGit.

Indicate that if you make changes to libgit2 that you must distribute
the source _to libgit2_, not the source _of your program_.

The old pthread-file did re-implement the pthreads API with exact symbol
matching. As the thread-abstraction has now been split up between Unix- and
Windows-specific files within the `git_` namespace to avoid symbol-clashes
between libgit2 and pthreads, the rewritten wrappers have nothing to do with
pthreads anymore.

Rename the Windows-specific pthread-files to honor this change.

The function pthread_num_processors_np is currently unused and superseded by the
function `git_online_cpus`. Remove the function.

Fix truncation of SHA in error message for git_odb_read

HTTP authentication scheme name is case insensitive.

checkout: use empty baseline when no index file exists

documentation: improve docs for `checkout_head`

When no index file exists and a baseline is not explicitly provided, use
an empty baseline instead of trying to load `HEAD`.

`git_checkout_head` is sadly misunderstood as something that can
switch branches.  It cannot.  Update the documentation to reflect this.

fetch: Fixed spurious update callback for existing tags.

Memory leak fixes

Fix invalid memory references