Try to be more helpful when git-absorb does nothing or stops early

- Print "Please use --base to specify the base commit." when all the
  commits were "hidden" by other branches. (This makes the misbehavior
  described in tummychow#9 less confusing.)

- Instead of noting that "merge commit found", explain what it means to
  the user: "Will not fix up past the merge commit".

- Provide suggestions when "stack limit reached": "use --base or
  configure absorb.maxStack to override"

  - Don't honor absorb.maxStack with explicit --base

- Note if the stack is empty: "No commits available to fix up, exiting"

- Warn if "No additions staged, try adding something to the index."

- Print "Could not find a commit to fix up, use --base to increase the
  search range." instead of "could not find noncommutative commit", and
  display this message by default (without the verbose mode)

src/lib.rs

@@ -21,6 +21,12 @@ pub fn run(config: &Config) -> Result<(), failure::Error> {
     let repo = git2::Repository::open_from_env()?;
     debug!(config.logger, "repository found"; "path" => repo.path().to_str());
 
+    let stack = stack::working_stack(&repo, config.base, config.logger)?;
+    if stack.is_empty() {
+        crit!(config.logger, "No commits available to fix up, exiting");
+        return Ok(());
+    }
+
     let mut diff_options = Some({
         let mut ret = git2::DiffOptions::new();
         ret.context_lines(0)
@@ -31,7 +37,6 @@ pub fn run(config: &Config) -> Result<(), failure::Error> {
     });
 
     let (stack, summary_counts): (Vec<_>, _) = {
-        let stack = stack::working_stack(&repo, config.base, config.logger)?;
         let mut diffs = Vec::with_capacity(stack.len());
         for commit in &stack {
             let diff = owned::Diff::new(
@@ -73,6 +78,7 @@ pub fn run(config: &Config) -> Result<(), failure::Error> {
     let signature = repo.signature()?;
     let mut head_commit = repo.head()?.peel_to_commit()?;
 
+    let mut patches_considered = 0usize;
     'patch: for index_patch in index.iter() {
         let old_path = index_patch.new_path.as_slice();
         if index_patch.status != git2::Delta::Modified {
@@ -83,6 +89,8 @@ pub fn run(config: &Config) -> Result<(), failure::Error> {
             continue 'patch;
         }
 
+        patches_considered += 1;
+
         let mut preceding_hunks_offset = 0isize;
         let mut applied_hunks_offset = 0isize;
         'hunk: for index_hunk in &index_patch.hunks {
@@ -193,7 +201,8 @@ pub fn run(config: &Config) -> Result<(), failure::Error> {
                 // the hunk commutes with every commit in the stack,
                 // so there is no commit to absorb it into
                 None => {
-                    debug!(config.logger, "could not find noncommutative commit");
+                    warn!(config.logger, "Could not find a commit to fix up, use \
+                                          --base to increase the search range.");
                     continue 'hunk;
                 }
             };
@@ -232,6 +241,10 @@ pub fn run(config: &Config) -> Result<(), failure::Error> {
         }
     }
 
+    if patches_considered == 0 {
+        warn!(config.logger, "No additions staged, try adding something to the index.");
+    }
+
     Ok(())
 }
 

src/stack.rs

@@ -61,11 +61,13 @@ pub fn working_stack<'repo>(
     }
 
     let mut ret = Vec::new();
+    let mut commits_considered = 0usize;
     let sig = repo.signature()?;
     for rev in revwalk {
+        commits_considered += 1;
         let commit = repo.find_commit(rev?)?;
         if commit.parents().len() > 1 {
-            warn!(logger, "merge commit found"; "commit" => commit.id().to_string());
+            warn!(logger, "Will not fix up past the merge commit"; "commit" => commit.id().to_string());
             break;
         }
         if commit.author().name_bytes() != sig.name_bytes()
@@ -74,13 +76,21 @@ pub fn working_stack<'repo>(
             warn!(logger, "foreign author found"; "commit" => commit.id().to_string());
             break;
         }
-        if ret.len() == max_stack(repo) {
-            warn!(logger, "stack limit reached"; "limit" => ret.len());
+        if ret.len() == max_stack(repo) && user_provided_base.is_none() {
+            warn!(logger, "stack limit reached, use --base or configure absorb.maxStack to override";
+                  "limit" => ret.len());
             break;
         }
         debug!(logger, "commit pushed onto stack"; "commit" => commit.id().to_string());
         ret.push(commit);
     }
+    if commits_considered == 0 {
+        if user_provided_base.is_none() {
+            warn!(logger, "Please use --base to specify a base commit.");
+        } else {
+            warn!(logger, "Please try a different --base");
+        }
+    }
     Ok(ret)
 }