cli: Disallow patch checkout unless matching

Instead of quietly switching to an old commit, error out if we can't
checkout the patch without overwriting the existing branch.

radicle-cli/examples/rad-patch-checkout-force.md

@@ -68,11 +68,16 @@ Your branch is behind 'rad/patches/6ff4f09c1b5a81347981f59b02ef43a31a07cdae' by
 nothing to commit, working tree clean
 ```
 
-If Bob was to run `rad patch checkout` again, their branch would not
-update to the latest commit. This is because it sees that the branch
-already exists and does not want overwrite any changes. Bob can choose
-to use the `--force` (`-f`) flag to ensure that they are looking at
-the latest changes:
+If Bob was to run `rad patch checkout` again, it would error.
+This is because the branch already exists and `rad` does not want to
+overwrite any changes. Bob can choose to use the `--force` (`-f`) flag to
+ensure that they are looking at the latest changes:
+
+``` ~bob (fail)
+$ rad patch checkout 6ff4f09 --name alice-init
+✗ Performing checkout... <canceled>
+✗ Error: branch 'alice-init' already exists (use `--force` to overwrite)
+```
 
 ``` ~bob
 $ rad patch checkout 6ff4f09 -f --name alice-init

radicle-cli/examples/rad-patch-checkout.md

@@ -35,6 +35,15 @@ $ rad patch checkout 6ff4f09
 ✓ Branch patch/6ff4f09 setup to track rad/patches/6ff4f09c1b5a81347981f59b02ef43a31a07cdae
 ```
 
+Note that `rad patch checkout` can be used to switch to the patch branch
+as long as we haven't made changes to it.
+
+```
+$ git checkout master -q
+$ rad patch checkout 6ff4f09
+✓ Switched to branch patch/6ff4f09
+```
+
 Now, let's add a README too!
 
 ```
@@ -46,22 +55,6 @@ $ git commit --message "Add README, just for the fun"
  create mode 100644 README.md
 ```
 
-But maybe we first wanted to rebase `master` so we ended up being on
-that branch:
-
-``` (stderr)
-$ git checkout master
-Switched to branch 'master'
-```
-
-We can be safe in the knowledge that our changes on the
-`patch/6ff4f09` branch are still safe:
-
-```
-$ rad patch checkout 6ff4f09
-✓ Switched to branch patch/6ff4f09
-```
-
 We can now finish off the update:
 
 ``` (stderr)

radicle-cli/src/commands/patch/checkout.rs

@@ -54,13 +54,21 @@ pub fn run(
 
     let commit =
         match working.find_branch(patch_branch.as_str(), radicle::git::raw::BranchType::Local) {
-            Ok(branch) if !opts.force => branch.get().peel_to_commit()?,
-            Ok(branch) => {
+            Ok(branch) if opts.force => {
                 let commit = find_patch_commit(revision, stored, working)?;
                 let mut r = branch.into_reference();
                 r.set_target(commit.id(), &format!("force update '{patch_branch}'"))?;
                 commit
             }
+            Ok(branch) => {
+                let head = branch.get().peel_to_commit()?;
+                if head.id() != *revision.head() {
+                    anyhow::bail!(
+                        "branch '{patch_branch}' already exists (use `--force` to overwrite)"
+                    );
+                }
+                head
+            }
             Err(e) if radicle::git::is_not_found_err(&e) => {
                 let commit = find_patch_commit(revision, stored, working)?;
                 // Create patch branch and switch to it.