Recognize conflict markers that have no label

Git only writes the space after a marker when there is a label to write
after it, and the label can be empty: `git checkout -m` with the diff3
conflict style, for instance, has no name for the common ancestor, so it
writes a bare "|||||||" line.

pkg/gui/mergeconflicts/find_conflicts.go

@@ -108,10 +108,12 @@ func hasMarkerPrefix[T string | []byte](line T, markerChar byte, markerSize int)
 }
 
 // A start, ancestor or end marker is followed by a space and a label, e.g.
-// "<<<<<<< HEAD".
+// "<<<<<<< HEAD". The label can be missing though, in which case git doesn't
+// write the space either; `git checkout -m` with the diff3 conflict style does
+// that for the ancestor marker, for example.
 func isConflictMarker[T string | []byte](line T, markerChar byte, markerSize int) bool {
 	return hasMarkerPrefix(line, markerChar, markerSize) &&
-		len(line) > markerSize && line[markerSize] == ' '
+		(len(line) == markerSize || line[markerSize] == ' ')
 }
 
 // The marker separating the two sides of a conflict never has a label after it.

pkg/gui/mergeconflicts/find_conflicts_test.go

@@ -61,6 +61,19 @@ func TestDetermineLineType(t *testing.T) {
 			line:     "<<<<<<<<",
 			expected: NOT_A_MARKER,
 		},
+		// Markers without a label
+		{
+			line:     "<<<<<<<",
+			expected: START,
+		},
+		{
+			line:     "|||||||",
+			expected: ANCESTOR,
+		},
+		{
+			line:     ">>>>>>>",
+			expected: END,
+		},
 		{
 			line:       strings.Repeat("<", 32) + " HEAD",
 			markerSize: 32,
@@ -136,6 +149,10 @@ func TestFindConflictsAux(t *testing.T) {
 			content:  " <<<<<<< ",
 			expected: false,
 		},
+		{
+			content:  ">>>>>>>",
+			expected: true,
+		},
 		{
 			content:  "a\nb\nc\n<<<<<<< ",
 			expected: true,