find_repo: Clean up and simplify logic

Add GIT_REPOSITORY_OPEN_FROM_ENV flag to respect $GIT_* environment vars

Document GIT_REPOSITORY_OPEN_NO_DOTGIT and GIT_REPOSITORY_OPEN_FROM_ENV.

Include conflicts in `git_index_read_index`

Ensure that we include conflicts when calling `git_index_read_index`,
which will remove conflicts in the index that do not exist in the new
target, and will add conflicts from the new target.

Most of `git_index_read_index` is common to reading any iterator.
Refactor it out in case we want to implement `read_tree` in terms of it
in the future.

submodule: Try to fetch when update fails to find the target commit.

blame: increment reference count for origin's commit

When we create a blame origin, we try to look up the blob that is
to be blamed at a certain revision. When this lookup fails, e.g.
because the file did not exist at that certain revision, we fail
to create the blame origin and return `NULL`. The blame origin
that we have just allocated is thereby free'd with
`origin_decref`.

The `origin_decref` function does not only decrement reference
counts for the blame origin, though, but also for its commit and
blob. When this is done in the error case, we will cause an
uneven reference count for these objects. This may result in
hard-to-debug failures at seemingly unrelated code paths, where
we try to access these objects when they in fact have already
been free'd.

Fix the issue by refactoring `make_origin` such that we only
allocate the object after the only function that may fail so that
we do not have to call `origin_decref` at all. Also fix the
`pass_blame` function, which indirectly calls `make_origin`, to
free the commit when `make_origin` failed.

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.

find_repo had a complex loop and heavily nested conditionals, making it
difficult to follow.  Simplify this as much as possible:

- Separate assignments from conditionals.
- Check the complex loop condition in the only place it can change.
- Break out of the loop on error, rather than going through the rest of
  the loop body first.
- Handle error cases by immediately breaking, rather than nesting
  conditionals.
- Free repo_link unconditionally on the way out of the function, rather
  than in multiple places.
- Add more comments on the remaining complex steps.

git_repository_open_ext provides parameters for the start path, whether
to search across filesystems, and what ceiling directories to stop at.
git commands have standard environment variables and defaults for each
of those, as well as various other parameters of the repository. To
avoid duplicate environment variable handling in users of libgit2, add a
GIT_REPOSITORY_OPEN_FROM_ENV flag, which makes git_repository_open_ext
automatically handle the appropriate environment variables. Commands
that intend to act just like those built into git itself can use this
flag to get the expected default behavior.

git_repository_open_ext with the GIT_REPOSITORY_OPEN_FROM_ENV flag
respects $GIT_DIR, $GIT_DISCOVERY_ACROSS_FILESYSTEM,
$GIT_CEILING_DIRECTORIES, $GIT_INDEX_FILE, $GIT_NAMESPACE,
$GIT_OBJECT_DIRECTORY, and $GIT_ALTERNATE_OBJECT_DIRECTORIES.  In the
future, when libgit2 gets worktree support, git_repository_open_env will
also respect $GIT_WORK_TREE and $GIT_COMMON_DIR; until then,
git_repository_open_ext with this flag will error out if either
$GIT_WORK_TREE or $GIT_COMMON_DIR is set.

GIT_REPOSITORY_OPEN_NO_SEARCH does not search up through parent
directories, but still tries the specified path both directly and with
/.git appended.  GIT_REPOSITORY_OPEN_BARE avoids appending /.git, but
opens the repository in bare mode even if it has a working directory.
To support the semantics git uses when given $GIT_DIR in the
environment, provide a new GIT_REPOSITORY_OPEN_NO_DOTGIT flag to not try
appending /.git.

git only checks ceiling directories when its search ascends to a parent
directory.  A ceiling directory matching the starting directory will not
prevent git from finding a repository in the starting directory or a
parent directory.  libgit2 handled the former case correctly, but
differed from git in the latter case: given a ceiling directory matching
the starting directory, but no repository at the starting directory,
libgit2 would stop the search at that point rather than finding a
repository in a parent directory.

Test case using git command-line tools:

/tmp$ git init x
Initialized empty Git repository in /tmp/x/.git/
/tmp$ cd x/
/tmp/x$ mkdir subdir
/tmp/x$ cd subdir/
/tmp/x/subdir$ GIT_CEILING_DIRECTORIES=/tmp/x git rev-parse --git-dir
fatal: Not a git repository (or any of the parent directories): .git
/tmp/x/subdir$ GIT_CEILING_DIRECTORIES=/tmp/x/subdir git rev-parse --git-dir
/tmp/x/.git

Fix the testsuite to test this case (in one case fixing a test that
depended on the current behavior), and then fix find_repo to handle this
case correctly.

In the process, simplify and document the logic in find_repo():
- Separate the concepts of "currently checking a .git directory" and
  "number of iterations left before going further counts as a search"
  into two separate variables, in_dot_git and min_iterations.
- Move the logic to handle in_dot_git and append /.git to the top of the
  loop.
- Only search ceiling_dirs and find ceiling_offset after running out of
  min_iterations; since ceiling_offset only tracks the longest matching
  ceiling directory, if ceiling_dirs contained both the current
  directory and a parent directory, this change makes find_repo stop the
  search at the parent directory.

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.