cli: Check for branch on checkout

The `rad patch checkout` command would blindly create a branch when
being executed. If there was a branch already existing and user had
committed to that branch, switched without updating the patch, and
then switched back, then they would lose the commit on that branch --
the original commit of the patch would be taken instead.

Fix this by first checking if the branch existed beforehand, and
setting the workspace to use that branch's commit.

Add a test to ensure that this works as expected.

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

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

@@ -0,0 +1,72 @@
+We may want to work on top of an existing patch and this where `rad
+patch checkout` comes into play. So, first we will create a patch to
+set up the workflow.
+
+```
+$ git checkout -b flux-capacitor-power
+$ touch REQUIREMENTS
+```
+
+Here the instructions are added to the project's README for 1.21 gigawatts and
+commit the changes to git.
+
+```
+$ git add REQUIREMENTS
+$ git commit -v -m "Define power requirements"
+[flux-capacitor-power 3e674d1] Define power requirements
+ 1 file changed, 0 insertions(+), 0 deletions(-)
+ create mode 100644 REQUIREMENTS
+```
+
+Once the code is ready, we open (or create) a patch with our changes for the project.
+
+``` (stderr)
+$ git push rad -o patch.message="Define power requirements" -o patch.message="See details." HEAD:refs/patches
+✓ Patch a8926643a8f6a65bc386b0131621994000485d4d opened
+To rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji/z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi
+ * [new reference]   HEAD -> refs/patches
+```
+
+Now, let's checkout the patch that we just created:
+
+```
+$ rad patch checkout a892664
+✓ Switched to branch patch/a892664
+✓ Branch patch/a892664 setup to track rad/patches/a8926643a8f6a65bc386b0131621994000485d4d
+```
+
+Now, let's add a README too!
+
+```
+$ touch README.md
+$ git add README.md
+$ git commit --message "Add README, just for the fun"
+[patch/a892664 27857ec] Add README, just for the fun
+ 1 file changed, 0 insertions(+), 0 deletions(-)
+ 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/a892664` branch are still safe:
+
+```
+$ rad patch checkout a892664
+✓ Switched to branch patch/a892664
+```
+
+We can now finish off the update:
+
+``` (stderr)
+$ git push rad -o patch.message="Add README, just for the fun"
+✓ Patch a892664 updated to 8d8aa0887a11f2a37fa8ed0d5723efa96fd727ed
+To rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji/z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi
+   3e674d1..27857ec  patch/a892664 -> patches/a8926643a8f6a65bc386b0131621994000485d4d
+```

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

@@ -34,11 +34,20 @@ pub fn run(
     let patch_branch =
         // SAFETY: Patch IDs are valid refstrings.
         git::refname!("patch").join(RefString::try_from(term::format::cob(&patch_id)).unwrap());
-    let commit = find_patch_commit(revision, stored, working)?;
 
-    // Create patch branch and switch to it.
-    working.branch(patch_branch.as_str(), &commit, true)?;
-    working.checkout_tree(commit.as_object(), None)?;
+    match working.find_branch(patch_branch.as_str(), radicle::git::raw::BranchType::Local) {
+        Ok(branch) => {
+            let commit = branch.get().peel_to_commit()?;
+            working.checkout_tree(commit.as_object(), None)?;
+        }
+        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.
+            working.branch(patch_branch.as_str(), &commit, true)?;
+            working.checkout_tree(commit.as_object(), None)?;
+        }
+        Err(e) => return Err(e.into()),
+    }
     working.set_head(&git::refs::workdir::branch(&patch_branch))?;
 
     spinner.message(format!(

radicle-cli/tests/commands.rs

@@ -410,6 +410,26 @@ fn rad_patch() {
     test("examples/rad-patch.md", working.path(), Some(home), []).unwrap();
 }
 
+#[test]
+fn rad_patch_checkout() {
+    let mut environment = Environment::new();
+    let profile = environment.profile("alice");
+    let working = tempfile::tempdir().unwrap();
+    let home = &profile.home;
+
+    // Setup a test repository.
+    fixtures::repository(working.path());
+
+    test("examples/rad-init.md", working.path(), Some(home), []).unwrap();
+    test(
+        "examples/rad-patch-checkout.md",
+        working.path(),
+        Some(home),
+        [],
+    )
+    .unwrap();
+}
+
 #[test]
 fn rad_patch_update() {
     let mut environment = Environment::new();