fix(squash): preserve consensus author

When all of the patches to be squashed have the same author, use that
author for the squashed commit.

If the patches to squash have heterogeneous authors, default to the
configured user for the repo. (This was the behavior for all squashes.)

The command line flags (--author, --authname, --authemail) still take
precedence.

Fixes: #490

src/cmd/squash.rs

@@ -222,11 +222,17 @@ fn try_squash(
 ) -> Result<Option<(PatchName, gix::ObjectId)>> {
     let repo = trans.repo();
     let base_commit = trans.get_patch_commit(&patchnames[0]);
+    let base_author = base_commit.author()?;
+    let mut use_base_author = true;
     let base_commit_ref = base_commit.decode()?;
     if let Some(tree_id) = repo.stupid().with_temp_index(|stupid_temp| {
         stupid_temp.read_tree(base_commit_ref.tree())?;
         for commit in patchnames[1..].iter().map(|pn| trans.get_patch_commit(pn)) {
             let commit_ref = commit.decode()?;
+            let author = commit.author()?;
+            if author != base_author {
+                use_base_author = false;
+            }
             let parent = commit.get_parent_commit()?;
             let parent_commit_ref = parent.decode()?;
             if parent_commit_ref.tree() != commit_ref.tree()
@@ -259,7 +265,14 @@ fn try_squash(
             .allow_template_save(false)
             .template_patchname(patchname)
             .extra_allowed_patchnames(patchnames)
-            .default_author(repo.get_author()?.override_author(matches))
+            .default_author(
+                if use_base_author {
+                    base_author
+                } else {
+                    repo.get_author()?
+                }
+                .override_author(matches),
+            )
             .default_message(prepare_message(trans, patchnames)?)
             .edit(trans, repo, matches)?
         {

t/t2600-squash.sh

@@ -71,6 +71,54 @@ test_expect_success 'Squash at stack top' '
     [ "$(echo $(stg series --unapplied --noprefix))" = "" ]
 '
 
+test_expect_success 'Squash patches with all non-default author' '
+    echo "a" >>baz.txt &&
+    stg new -rm "a-patch" --author "Other Contributor <another@example.com>" &&
+    echo "b" >>baz.txt &&
+    stg new -rm "b-patch" --author "Other Contributor <another@example.com>" &&
+    echo "c" >>baz.txt &&
+    stg new -rm "c-patch" --author "Other Contributor <another@example.com>" &&
+    stg squash -m "abc-patch" a-patch b-patch c-patch &&
+    test_when_finished "stg delete abc-patch" &&
+    stg show abc-patch | grep "Author:" >out &&
+    cat >expected <<-\EOF &&
+	Author: Other Contributor <another@example.com>
+	EOF
+    test_cmp expected out
+'
+
+test_expect_success 'Squash patches with some non-default author' '
+    echo "a" >>baz.txt &&
+    stg new -rm "a-patch" &&
+    echo "b" >>baz.txt &&
+    stg new -rm "b-patch" --author "Other Contributor <another@example.com>" &&
+    echo "c" >>baz.txt &&
+    stg new -rm "c-patch" &&
+    stg squash -m "abc-patch" a-patch b-patch c-patch &&
+    test_when_finished "stg delete abc-patch" &&
+    stg show abc-patch | grep "Author:" >out &&
+    cat >expected <<-\EOF &&
+	Author: A Ú Thor <author@example.com>
+	EOF
+    test_cmp expected out
+'
+
+test_expect_success 'Squash patches with author override' '
+    echo "a" >>baz.txt &&
+    stg new -rm "a-patch" --author "Other Contributor <another@example.com>" &&
+    echo "b" >>baz.txt &&
+    stg new -rm "b-patch" --author "Other Contributor <another@example.com>" &&
+    echo "c" >>baz.txt &&
+    stg new -rm "c-patch" --author "Other Contributor <another@example.com>" &&
+    stg squash -m "abc-patch" --author "Override Author <override@example.com>" a-patch b-patch c-patch &&
+    test_when_finished "stg delete abc-patch" &&
+    stg show abc-patch | grep "Author:" >out &&
+    cat >expected <<-\EOF &&
+	Author: Override Author <override@example.com>
+	EOF
+    test_cmp expected out
+'
+
 test_expect_success 'Empty commit message aborts the squash' '
     write_script fake-editor <<-\EOF &&
 	echo "" >"$1"