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

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

The thread local storage is used to hold some global state that
is dynamically allocated and should be freed upon exit. On
Windows, we clean up the C run-time right after execution of
registered shutdown callbacks and before cleaning up the TLS.

When we clean up the CRT, we also cause it to analyze for memory
leaks. As we did not free the TLS yet this will lead to false
positives.

Fix the issue by first freeing the TLS and cleaning up the CRT
only afterwards.

When removing an entry from the index by its position, we first
retrieve the position from the index's entries and then try to
remove the retrieved value from the index map with
`DELETE_IN_MAP`. When `index_remove_entry` returns `NULL` we try
to feed it into the `DELETE_IN_MAP` macro, which will
unconditionally call `idxentry_hash` and then happily dereference
the `NULL` entry pointer.

Fix the issue by not passing a `NULL` entry into `DELETE_IN_MAP`.

When we receive a packet of exactly four bytes encoding its
length as those four bytes it can be treated as an empty line.
While it is not really specified how those empty lines should be
treated, we currently ignore them and do not return an error when
trying to parse it but simply advance the data pointer.

Callers invoking `git_pkt_parse_line` are currently not prepared
to handle this case as they do not explicitly check this case.
While they could always reset the passed out-pointer to `NULL`
before calling `git_pkt_parse_line` and determine if the pointer
has been set afterwards, it makes more sense to update
`git_pkt_parse_line` to set the out-pointer to `NULL` itself when
it encounters such an empty packet. Like this it is guaranteed
that there will be no invalid memory references to free'd
pointers.

As such, the issue has been fixed such that `git_pkt_parse_line`
always sets the packet out pointer to `NULL` when an empty packet
has been received and callers check for this condition, skipping
such packets.

`git_index_read_index` fixes

When adding a new entry to an existing index via `git_index_read_index`,
be sure to remove the tree cache entry for that new path.  This will
mark all parent trees as dirty.

Test a rebase (both a merge rebase and an inmemory rebase) with a new
commit that adds files underneath a new subfolder.

Read a tree into an index, write the index, then re-open the index and
ensure that we are treesame to the original.