cli: Allow setting a different patch base on push

Using `-o patch.base=<oid>`, users can specify a different patch base,
when creating or updating a patch.

radicle-cli/examples/rad-patch-ahead-behind.md

@@ -115,6 +115,40 @@ $ rad patch show -v 57cb9b2758518e547de324456ac967fda456c6c1
 │ 7f63fcb Add Mel                                                    │
 │ 5c88a79 Add Alan                                                   │
 ├────────────────────────────────────────────────────────────────────┤
-│ ● opened by (you) (z6MknSL…StBU8Vi) 5 months ago                   │
+│ ● opened by (you) (z6MknSL…StBU8Vi) [            ...             ] │
+╰────────────────────────────────────────────────────────────────────╯
+```
+
+If we want to instead create a "stacked" patch, we can do so with the
+`patch.base` push option:
+
+``` (stderr)
+$ git push -o patch.message="Add Mel #2" -o patch.base=5c88a79d75f5c2b4cc51ee6f163d2db91ee198d7 rad HEAD:refs/patches
+✓ Patch 395221c5b75fa9bc2de7909d03e69dfd606611c6 opened
+To rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji/z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi
+ * [new reference]   HEAD -> refs/patches
+```
+
+As you'll notice, using the previous patch as the base, we only see commit
+`7f63fcb` listed for this new patch.
+
+However, since the patch is still intended to be merged into `master`, we see
+that it is still two commits ahead and one behind from `master`.
+
+```
+$ rad patch show -v 395221c5b75fa9bc2de7909d03e69dfd606611c6
+╭────────────────────────────────────────────────────────────────────╮
+│ Title     Add Mel #2                                               │
+│ Patch     395221c5b75fa9bc2de7909d03e69dfd606611c6                 │
+│ Author    did:key:z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi │
+│ Head      7f63fcbcf23fc39eea784c091ad3d20d7e4bd005                 │
+│ Base      5c88a79d75f5c2b4cc51ee6f163d2db91ee198d7                 │
+│ Branches  feature/2                                                │
+│ Commits   ahead 2, behind 1                                        │
+│ Status    open                                                     │
+├────────────────────────────────────────────────────────────────────┤
+│ 7f63fcb Add Mel                                                    │
+├────────────────────────────────────────────────────────────────────┤
+│ ● opened by (you) (z6MknSL…StBU8Vi) [            ...             ] │
 ╰────────────────────────────────────────────────────────────────────╯
 ```

radicle-remote-helper/src/lib.rs

@@ -60,6 +60,8 @@ pub struct Options {
     no_sync: bool,
     /// Open patch in draft mode.
     draft: bool,
+    /// Patch base to use, when opening or updating a patch.
+    base: Option<git::Oid>,
     /// Patch message.
     message: cli::patch::Message,
 }
@@ -115,8 +117,18 @@ pub fn run(profile: radicle::Profile) -> Result<(), Error> {
                         let args = args.join(" ");
 
                         if let Some((key, val)) = args.split_once('=') {
-                            if key == "patch.message" {
-                                opts.message.append(val);
+                            match key {
+                                "patch.message" => {
+                                    opts.message.append(val);
+                                }
+                                "patch.base" => {
+                                    let base = val.parse()?;
+                                    opts.base = Some(base);
+                                }
+                                _ => {
+                                    println!("unsupported");
+                                    continue;
+                                }
                             }
                         } else {
                             println!("unsupported");

radicle-remote-helper/src/push.rs

@@ -50,6 +50,9 @@ pub enum Error {
     /// Git error.
     #[error("git: {0}")]
     Git(#[from] git::raw::Error),
+    /// Git extension error.
+    #[error("git: {0}")]
+    GitExt(#[from] git::ext::Error),
     /// Storage error.
     #[error(transparent)]
     Storage(#[from] radicle::storage::Error),
@@ -261,16 +264,17 @@ fn patch_open<G: Signer>(
     push_ref(src, &dst, false, working, stored.raw())?;
 
     let (_, target) = stored.canonical_head()?;
-    let base = stored.backend.merge_base(*target, commit.id())?;
-    if base == commit.id() {
+    let head = commit.id().into();
+    let base = if let Some(base) = opts.base {
+        base
+    } else {
+        stored.merge_base(&target, &head)?
+    };
+    if base == head {
         return Err(Error::EmptyPatch);
     }
-    let (title, description) = cli::patch::get_create_message(
-        opts.message,
-        &stored.backend,
-        &base.into(),
-        &commit.id().into(),
-    )?;
+    let (title, description) =
+        cli::patch::get_create_message(opts.message, &stored.backend, &base, &head)?;
 
     let mut patches = patch::Patches::open(stored)?;
     let patch = if opts.draft {
@@ -401,8 +405,16 @@ fn patch_update<G: Signer>(
     )?;
 
     let (_, target) = stored.canonical_head()?;
-    let base = stored.backend.merge_base(*target, commit.id())?;
-    let revision = patch.update(message, base, commit.id(), signer)?;
+    let head: git::Oid = commit.id().into();
+    let base = if let Some(base) = opts.base {
+        base
+    } else {
+        stored.merge_base(&target, &head)?
+    };
+    if base == head {
+        return Err(Error::EmptyPatch);
+    }
+    let revision = patch.update(message, base, head, signer)?;
 
     eprintln!(
         "{} Patch {} updated to {}",

radicle/src/cob/patch.rs

@@ -305,12 +305,7 @@ impl Patch {
 
     /// Get the merge base of this patch.
     pub fn merge_base<R: ReadRepository>(&self, repo: &R) -> Result<git::Oid, git::ext::Error> {
-        let target = self.target().head(repo).map_err(|_| {
-            git::ext::Error::NotFound(git::ext::NotFound::NoSuchBranch(String::from("HEAD")))
-        })?;
-        let base = repo.merge_base(&target, self.head())?;
-
-        Ok(base)
+        repo.merge_base(self.base(), self.head())
     }
 
     /// Get the commit range of this patch.