localrepo: Parse `MergeTree` errors only for exit status 1

The manual page for git-merge-tree(1) states that the exit status is set
to 1, only when there is conflicts. If there are other errors, the exit
status is neither 0 (which is used to denote no conflicts) or 1.

In the `MergeTree` however we default to parsing conflict errors when
the status is not >1. This shouldn't matter as long as `ExecAndWait()`
doesn't return an exist status of 0 plus an error. But is confusing
nevertheless. Let's rewrite the code to explicitly check for exit status
1.

1 files changed, 7 insertions, 6 deletions

diff --git a/internal/git/localrepo/merge.go b/internal/git/localrepo/merge.go
index 4b09f1b01..6cc922357 100644
--- a/internal/git/localrepo/merge.go
+++ b/internal/git/localrepo/merge.go
@@ -104,14 +104,15 @@ func (repo *Repo) MergeTree(
 			return "", errors.New("could not parse exit status of merge-tree(1)")
 		}
 
-		if exitCode > 1 {
-			if text.ChompBytes(stderr.Bytes()) == "fatal: refusing to merge unrelated histories" {
-				return "", ErrMergeTreeUnrelatedHistory
-			}
-			return "", fmt.Errorf("merge-tree: %w", err)
+		if exitCode == 1 {
+			return parseMergeTreeError(objectHash, config, stdout.String())
+		}
+
+		if text.ChompBytes(stderr.Bytes()) == "fatal: refusing to merge unrelated histories" {
+			return "", ErrMergeTreeUnrelatedHistory
 		}
 
-		return parseMergeTreeError(objectHash, config, stdout.String())
+		return "", fmt.Errorf("merge-tree: %w", err)
 	}
 
 	oid, err := objectHash.FromHex(strings.Split(stdout.String(), "\x00")[0])