cli: hint about how to recover from failed export of branches

src/cli_util.rs

@@ -907,6 +907,13 @@ pub fn print_failed_git_export(
             formatter.with_label("branch", |formatter| formatter.write_str(branch_name))?;
             formatter.write_str("\n")?;
         }
+        drop(formatter);
+        ui.write_hint(
+            r#"Hint: Git doesn't allow a branch name that looks like a parent directory of
+another (e.g. `foo` and `foo/bar`). Try to rename the branches that failed to
+export or their "parent" branches.
+"#,
+        )?;
     }
     Ok(())
 }

tests/test_git_colocated.rs

@@ -193,6 +193,9 @@ fn test_git_colocated_conflicting_git_refs() {
     insta::assert_snapshot!(get_stderr_string(&assert), @r###"
     Failed to export some branches:
       main/sub
+    Hint: Git doesn't allow a branch name that looks like a parent directory of
+    another (e.g. `foo` and `foo/bar`). Try to rename the branches that failed to
+    export or their "parent" branches.
     "###);
 }
 

tests/test_git_export.rs

@@ -32,5 +32,8 @@ fn test_git_export_conflicting_git_refs() {
     insta::assert_snapshot!(get_stderr_string(&assert), @r###"
     Failed to export some branches:
       main/sub
+    Hint: Git doesn't allow a branch name that looks like a parent directory of
+    another (e.g. `foo` and `foo/bar`). Try to rename the branches that failed to
+    export or their "parent" branches.
     "###);
 }