feat: pull and rebase record stack modifications

When `stg pull` and `stg rebase` perform their underlying `git pull` or
`git rebase`, the HEAD reference is typically moved. Previously, StGit
relied on a subsequent operation, typically `stg push`, to trigger the
"external modification" check at transaction execute time which would
record a new stack state that reflects the updated branch head.

For pull and rebase, these aren't really "external" modifications since
StGit is managing them.

StGit now records a new stack state immediately, and with a more
appropriate message when pull or rebase cause the branch head to move. This
makes the `stg log` clearer. It also allows a pull or rebase to be undone
with `stg undo` without having to first perform an intervening
stack-modifying command.

Author: jpgrayson

src/cmd/pull.rs

@@ -218,11 +218,17 @@ fn run(matches: &ArgMatches) -> Result<()> {
         stupid.user_rebase(&rebase_cmd, rebase_target)?;
     }
 
-    if !matches.get_flag("nopush") {
-        // The above pull and rebase action may have moved the stack's branch reference,
-        // so we initialize the stack afresh.
-        let stack = Stack::from_branch(&repo, None)?;
+    // The above pull and rebase action may have moved the stack's branch reference,
+    // so we initialize the stack afresh.
+    let stack = Stack::from_branch(&repo, None)?;
+    let stack = if stack.is_head_top() {
+        stack
+    } else {
+        // Record a new stack state with updated head since the pull moved the head.
+        stack.log_external_mods(Some("pull"))?
+    };
 
+    if !matches.get_flag("nopush") {
         stack.check_head_top_mismatch()?;
         let check_merged = matches.get_flag("merged");
         stack

src/cmd/rebase.rs

@@ -157,10 +157,17 @@ fn run(matches: &ArgMatches) -> Result<()> {
     print_info_message(matches, &format!("Rebasing to `{}`", target_commit.id()));
     stupid.user_rebase(&rebase_cmd, target_commit.id())?;
 
+    let stack = Stack::from_branch(&repo, None)?;
+    let stack = if stack.is_head_top() {
+        stack
+    } else {
+        // Record a new stack state with updated head since the head moved.
+        stack.log_external_mods(Some("rebase"))?
+    };
+
     if matches.get_flag("interactive") {
-        interactive_pushback(&repo, &config, matches, &applied)?;
+        interactive_pushback(stack, &repo, &config, matches, &applied)?;
     } else if !matches.get_flag("nopush") {
-        let stack = Stack::from_branch(&repo, None)?;
         stack.check_head_top_mismatch()?;
         let check_merged = matches.get_flag("merged");
         stack
@@ -219,12 +226,13 @@ enum Action {
 }
 
 fn interactive_pushback(
+    stack: Stack,
     repo: &git2::Repository,
     config: &git2::Config,
     matches: &ArgMatches,
     previously_applied: &[PatchName],
 ) -> Result<()> {
-    let mut stack = Stack::from_branch(repo, None)?;
+    let mut stack = stack;
 
     if stack.all_patches().next().is_none() {
         return Ok(());

src/stack/stack.rs

@@ -183,17 +183,19 @@ impl<'repo> Stack<'repo> {
     }
 
     /// Re-commit stack state with updated branch head.
-    pub fn log_external_mods(self) -> Result<Self> {
+    pub fn log_external_mods(self, message: Option<&str>) -> Result<Self> {
         let state_ref = self.repo.find_reference(&self.refname)?;
         let prev_state_commit = state_ref.peel_to_commit()?;
         let prev_state_commit_id = prev_state_commit.id();
         let state = self
             .state
             .advance_head(self.branch_head.clone(), prev_state_commit);
 
-        let message = "external modifications\n\
-                       \n\
-                       Modifications by tools other than StGit (e.g. git).\n";
+        let message = message.unwrap_or(
+            "external modifications\n\
+             \n\
+             Modifications by tools other than StGit (e.g. git).\n",
+        );
         let reflog_msg = "external modifications";
 
         let state_commit_id = state.commit(self.repo, None, message)?;

src/stack/transaction/mod.rs

@@ -153,7 +153,7 @@ impl<'repo> ExecuteContext<'repo> {
         let mut stack = if stack.is_head_top() {
             stack
         } else {
-            stack.log_external_mods()?
+            stack.log_external_mods(None)?
         };
 
         // Roll back by checking out the stack top tree prior to any changes from this

t/t2103-pull-trailing.sh

@@ -39,6 +39,19 @@ test_expect_success 'Port those patches to orig tree' '
     )
 '
 
+test_expect_success 'Undo pull operation' '
+    (
+        cd bar &&
+        stg id {base} >before-pull &&
+        stg pull --nopush &&
+        stg log -n1 | grep -e "pull$" &&
+        test_cmp_rev ! $(cat before-pull) $(stg id) &&
+        stg undo &&
+        stg id >after-undo &&
+        test_cmp before-pull after-undo
+    )
+'
+
 test_expect_success 'Pull those patches applied upstream, without pushing' '
     (
         cd bar &&