CHANGELOG.md: Document new flags for git_repository_open_ext
CHANGELOG.md: Document new flags for git_repository_open_ext

Document GIT_REPOSITORY_OPEN_NO_DOTGIT and GIT_REPOSITORY_OPEN_FROM_ENV.

Corillian/libgit2,Corillian/libgit2,stewid/libgit2,Corillian/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2,Corillian/libgit2,Corillian/libgit2,stewid/libgit2,stewid/libgit2,Corillian/libgit2