radicle: Improve cwd to search parents

If a `rad` command that requires an RID is executed within a
subdirectory of a Radicle project, then it will fail saying:

    ✗ Error: Current directory is not a radicle project

Improve this flow by searching parent directories and/or using the
GIT_DIR environment variable.

This is safe as long as the operations are read-only when calling
`cwd`. Currently they are:
- `Repository::open_ext` -- will not create a repository, only open
  one.
- `Repository::find_remote` -- this will only read the remote from the
  config and get the URL to parse the RID.

Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>
X-Clacks-Overhead: GNU Terry Pratchett

radicle/src/rad.rs

@@ -269,8 +269,23 @@ pub fn remove_remote(repo: &git2::Repository) -> Result<(), RemoteError> {
 }
 
 /// Get the Id of project in current working directory
+///
+/// It will atempt to search parent directories if `path` did not find
+/// a git repository.
+///
+/// # Safety
+///
+/// This function should only perform read operations since we do not
+/// want to modify the wrong repository in the case that it found a
+/// Git repository that is not a Radicle repository.
 pub fn cwd() -> Result<(git2::Repository, Id), RemoteError> {
-    let repo = git2::Repository::open(Path::new("."))?;
+    let mut flags = git2::RepositoryOpenFlags::empty();
+    // Allow to search upwards.
+    flags.set(git2::RepositoryOpenFlags::NO_SEARCH, false);
+    // Allow to use `GIT_DIR` env.
+    flags.set(git2::RepositoryOpenFlags::FROM_ENV, true);
+    let ceilings: &[&str] = &[];
+    let repo = git2::Repository::open_ext(Path::new("."), flags, ceilings)?;
     let (_, id) = remote(&repo)?;
 
     Ok((repo, id))