obslog: reverse order of predecessors in topo traversal

Currently, when there is a commit with two predecessors, the graph
splits into two branches, and all of the predecessors on the first
branch are printed before all of the predecessors on the second branch.
This causes the graph to grow wider with each squashed commit, since the
second branch must always get indented one level farther each time a
commit is squashed. I have some commits where the graph is indented more
than 10 levels due to squashing more than 10 times, making it very
difficult to read.

Reversing the order and printing the second branch before the first
branch prevents this unnecessary indentation and makes the graph easier
to read. This does not change the order of the edges in the graph (i.e.
the first predecessor is still the first edge and the second predecessor
is still the second edge in the graph).

cli/src/commands/obslog.rs

@@ -113,7 +113,21 @@ pub(crate) fn cmd_obslog(
     let mut commits = topo_order_reverse_ok(
         vec![Ok(start_commit)],
         |commit: &Commit| commit.id().clone(),
-        |commit: &Commit| commit.predecessors().collect_vec(),
+        |commit: &Commit| {
+            let mut predecessors = commit.predecessors().collect_vec();
+            // Predecessors don't need to follow any defined order. However in
+            // practice, if there are multiple predecessors, then usually the
+            // first predecessor is the previous version of the same change, and
+            // the other predecessors are commits that were squashed into it. If
+            // multiple commits are squashed at once, then they are usually
+            // recorded in chronological order. We want to show squashed commits
+            // in reverse chronological order, and we also want to show squashed
+            // commits before the squash destination (since the destination's
+            // subgraph may contain earlier squashed commits as well), so we
+            // visit the predecessors in reverse order.
+            predecessors.reverse();
+            predecessors
+        },
     )?;
     if args.deprecated_limit.is_some() {
         writeln!(

cli/tests/test_obslog_command.rs

@@ -252,36 +252,36 @@ fn test_obslog_squash() {
     │ │     1    1: foo
     │ │     2    2: bar
     │ │          3: baz
-    ◉ │    qpvuntsm hidden test.user@example.com 2001-02-03 08:05:10 e3c2a446
-    ├───╮  squashed 1
-    │ │ │  Modified regular file file1:
-    │ │ │     1    1: foo
-    │ │ │          2: bar
-    ◉ │ │  qpvuntsm hidden test.user@example.com 2001-02-03 08:05:09 766420db
-    │ │ │  first
-    │ │ │  Added regular file file1:
-    │ │ │          1: foo
-    ◉ │ │  qpvuntsm hidden test.user@example.com 2001-02-03 08:05:08 fa15625b
-    │ │ │  (empty) first
-    ◉ │ │  qpvuntsm hidden test.user@example.com 2001-02-03 08:05:07 230dd059
-      │ │  (empty) (no description set)
-      │ ◉  kkmpptxz hidden test.user@example.com 2001-02-03 08:05:10 46acd22a
-      │ │  second
-      │ │  Modified regular file file1:
-      │ │     1    1: foo
-      │ │          2: bar
-      │ ◉  kkmpptxz hidden test.user@example.com 2001-02-03 08:05:09 cba41deb
-      │    (empty) second
-      ◉  zsuskuln hidden test.user@example.com 2001-02-03 08:05:12 7015a42c
-      │  third
-      │  Modified regular file file1:
-      │     1    1: foo
-      │     2    2: bar
-      │          3: baz
-      ◉  zsuskuln hidden test.user@example.com 2001-02-03 08:05:11 66645763
-      │  (empty) third
-      ◉  zsuskuln hidden test.user@example.com 2001-02-03 08:05:10 1c7afcb4
-         (empty) (no description set)
+    │ ◉  zsuskuln hidden test.user@example.com 2001-02-03 08:05:12 7015a42c
+    │ │  third
+    │ │  Modified regular file file1:
+    │ │     1    1: foo
+    │ │     2    2: bar
+    │ │          3: baz
+    │ ◉  zsuskuln hidden test.user@example.com 2001-02-03 08:05:11 66645763
+    │ │  (empty) third
+    │ ◉  zsuskuln hidden test.user@example.com 2001-02-03 08:05:10 1c7afcb4
+    │    (empty) (no description set)
+    ◉    qpvuntsm hidden test.user@example.com 2001-02-03 08:05:10 e3c2a446
+    ├─╮  squashed 1
+    │ │  Modified regular file file1:
+    │ │     1    1: foo
+    │ │          2: bar
+    │ ◉  kkmpptxz hidden test.user@example.com 2001-02-03 08:05:10 46acd22a
+    │ │  second
+    │ │  Modified regular file file1:
+    │ │     1    1: foo
+    │ │          2: bar
+    │ ◉  kkmpptxz hidden test.user@example.com 2001-02-03 08:05:09 cba41deb
+    │    (empty) second
+    ◉  qpvuntsm hidden test.user@example.com 2001-02-03 08:05:09 766420db
+    │  first
+    │  Added regular file file1:
+    │          1: foo
+    ◉  qpvuntsm hidden test.user@example.com 2001-02-03 08:05:08 fa15625b
+    │  (empty) first
+    ◉  qpvuntsm hidden test.user@example.com 2001-02-03 08:05:07 230dd059
+       (empty) (no description set)
     "###);
 }
 

cli/tests/test_squash_command.rs

@@ -972,10 +972,10 @@ fn test_squash_from_multiple_partial_no_op() {
     insta::assert_snapshot!(stdout, @r###"
     @    e178068add8c d
     ├─╮
-    ◉ │  b37ca1ee3306 d
-    ◉ │  1d9eb34614c9 d
-      ◉  b73077b08c59 b
-      ◉  a786561e909f b
+    │ ◉  b73077b08c59 b
+    │ ◉  a786561e909f b
+    ◉  b37ca1ee3306 d
+    ◉  1d9eb34614c9 d
     "###);
 
     // If no source commits match the paths, then the whole operation is a no-op