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.

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_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.

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.