Option flags for git_repository_open_ext.

• GIT_REPOSITORY_OPEN_NO_SEARCH - Only open the repository if it can be immediately found in the start_path. Do not walk up from the start_path looking at parent directories.
• GIT_REPOSITORY_OPEN_CROSS_FS - Unless this flag is set, open will not continue searching across filesystem boundaries (i.e. when st_dev changes from the stat system call). (E.g. Searching in a user's home directory "/home/user/source/" will not return "/.git/" as the found repo if "/" is a different filesystem than "/home".)
• GIT_REPOSITORY_OPEN_BARE - Open repository as a bare repo regardless of core.bare config, and defer loading config file for faster setup. Unlike git_repository_open_bare, this can follow gitlinks.
• GIT_REPOSITORY_OPEN_NO_DOTGIT - Do not check for a repository by appending /.git to the start_path; only open the repository if start_path itself points to the git directory.
• GIT_REPOSITORY_OPEN_FROM_ENV - Find and open a git repository, respecting the environment variables used by the git command-line tools. If set, git_repository_open_ext will ignore the other flags and the ceiling_dirs argument, and will allow a NULL path to use GIT_DIR or search from the current directory. The search for a repository will respect $GIT_CEILING_DIRECTORIES and $GIT_DISCOVERY_ACROSS_FILESYSTEM. The opened repository will respect $GIT_INDEX_FILE, $GIT_NAMESPACE, $GIT_OBJECT_DIRECTORY, and $GIT_ALTERNATE_OBJECT_DIRECTORIES. In the future, this flag will also cause git_repository_open_ext to respect $GIT_WORK_TREE and $GIT_COMMON_DIR; currently, git_repository_open_ext with this flag will error out if either $GIT_WORK_TREE or $GIT_COMMON_DIR is set.