Add :prune=trivial-merge filter

docs/src/reference/filters.md

@@ -114,6 +114,13 @@ commits that don't match any of the other shas.
 Produce the history that would be the result of pushing the passed branches with the
 passed filters into the upstream.
 
+### Prune trivial merge commits **:prune=trivial-merge**
+
+Produce a history that skips all merge commits whose tree is identical to the first parents
+tree.
+Normally Josh will keep all commits in the filtered history whose tree differs from any of it's
+parents.
+
 Filter order matters
 --------------------
 

josh-core/src/filter/mod.rs

@@ -144,6 +144,7 @@ enum Op {
     Rev(std::collections::BTreeMap<LazyRef, Filter>),
     Join(std::collections::BTreeMap<LazyRef, Filter>),
     Linear,
+    Prune,
     Unsign,
 
     RegexReplace(Vec<(regex::Regex, String)>),
@@ -485,6 +486,7 @@ fn spec2(op: &Op) -> String {
         Op::Unsign => ":unsign".to_string(),
         Op::Subdir(path) => format!(":/{}", parse::quote_if(&path.to_string_lossy())),
         Op::File(path) => format!("::{}", parse::quote_if(&path.to_string_lossy())),
+        Op::Prune => ":prune=trivial-merge".to_string(),
         Op::Prefix(path) => format!(":prefix={}", parse::quote_if(&path.to_string_lossy())),
         Op::Pattern(pattern) => format!("::{}", parse::quote_if(pattern)),
         Op::Author(author, email) => {
@@ -811,6 +813,33 @@ fn apply_to_commit2(
             ))
             .transpose();
         }
+        Op::Prune => {
+            let p: Vec<_> = commit.parent_ids().collect();
+
+            if p.len() > 0 {
+                let parent = some_or!(transaction.get(filter, p[0]), {
+                    return Ok(None);
+                });
+
+                let parent_tree = transaction.repo().find_commit(parent)?.tree_id();
+
+                if parent_tree == commit.tree_id() {
+                    return Ok(Some(history::drop_commit(
+                        commit,
+                        vec![parent],
+                        transaction,
+                        filter,
+                    )?));
+                }
+            }
+
+            RewriteData {
+                tree: commit.tree()?,
+                message: None,
+                author: None,
+                committer: None,
+            }
+        }
         Op::Unsign => {
             let parents: Vec<_> = commit.parent_ids().collect();
 
@@ -998,6 +1027,7 @@ fn apply2<'a>(
         Op::Committer(_, _) => Ok(tree),
         Op::Squash(Some(_)) => Err(josh_error("not applicable to tree")),
         Op::Linear => Ok(tree),
+        Op::Prune => Ok(tree),
         Op::Unsign => Ok(tree),
         Op::Rev(_) => Err(josh_error("not applicable to tree")),
         Op::Join(_) => Err(josh_error("not applicable to tree")),

josh-core/src/filter/opt.rs

@@ -489,6 +489,7 @@ pub fn invert(filter: Filter) -> JoshResult<Filter> {
     let result = match to_op(filter) {
         Op::Nop => Some(Op::Nop),
         Op::Linear => Some(Op::Nop),
+        Op::Prune => Some(Op::Prune),
         Op::Unsign => Some(Op::Unsign),
         Op::Empty => Some(Op::Empty),
         Op::Subdir(path) => Some(Op::Prefix(path)),

josh-core/src/filter/parse.rs

@@ -35,6 +35,22 @@ fn make_op(args: &[&str]) -> JoshResult<Op> {
         ["SQUASH"] => Ok(Op::Squash(None)),
         ["SQUASH", _ids @ ..] => Err(josh_error("SQUASH with ids can't be parsed")),
         ["linear"] => Ok(Op::Linear),
+        ["prune", "trivial-merge"] => Ok(Op::Prune),
+        ["prune"] => Err(josh_error(indoc!(
+            r#"
+            Filter ":prune" requires an argument.
+
+            Note: use "=" to provide the argument value:
+
+              :prune=trivial-merge
+            "#
+        ))),
+        ["prune", _] => Err(josh_error(indoc!(
+            r#"
+            Filter ":prune" only supports "trivial-merge"
+            as arguement value.
+            "#
+        ))),
         ["unsign"] => Ok(Op::Unsign),
         ["PATHS"] => Ok(Op::Paths),
         ["INDEX"] => Ok(Op::Index),

tests/filter/prune_trivial_merge.t

@@ -0,0 +1,47 @@
+  $ export RUST_BACKTRACE=1
+  $ git init -q 1> /dev/null
+
+  $ echo contents1 > file1
+  $ git add .
+  $ git commit -m "add file1" 1> /dev/null
+
+  $ git log --graph --pretty=%s
+  * add file1
+
+  $ git checkout -b branch1
+  Switched to a new branch 'branch1'
+  $ echo contents2 > file2
+  $ git add .
+  $ git commit -m "add file2" 1> /dev/null
+
+  $ git checkout master
+  Switched to branch 'master'
+
+  $ echo contents3 > file1
+  $ git add .
+  $ git commit -m "mod file1" 1> /dev/null
+
+  $ git merge -q  branch1 --no-ff
+  $ git log --graph --pretty=%s
+  *   Merge branch 'branch1'
+  |\  
+  | * add file2
+  * | mod file1
+  |/  
+  * add file1
+
+  $ josh-filter -s ::file1
+  [3] ::file1
+  $ git log --graph --pretty=%s FILTERED_HEAD
+  *   Merge branch 'branch1'
+  |\  
+  * | mod file1
+  |/  
+  * add file1
+  $ josh-filter -s ::file1:prune=trivial-merge
+  [2] :prune=trivial-merge
+  [3] ::file1
+
+  $ git log --graph --pretty=%s FILTERED_HEAD
+  * mod file1
+  * add file1