command/format: Fix empty overlap diagnostics

Diagnostics where the highlight range has an empty overlap with a line
would skip lines of the output. This is because if two ranges abut each
other, they can be considered to overlap, but that overlap is empty.
This results in an edge case in the diagnostic printer which causes the
line not to be printed.

command/format/diagnostic.go

@@ -100,7 +100,7 @@ func Diagnostic(diag tfdiags.Diagnostic, sources map[string][]byte, color *color
 				if !lineRange.Overlaps(snippetRange) {
 					continue
 				}
-				if lineRange.Overlaps(highlightRange) {
+				if !lineRange.Overlap(highlightRange).Empty() {
 					beforeRange, highlightedRange, afterRange := lineRange.PartitionAround(highlightRange)
 					before := beforeRange.SliceBytes(src)
 					highlighted := highlightedRange.SliceBytes(src)

command/format/diagnostic_test.go

@@ -120,3 +120,50 @@ Error: Some error
 		t.Fatalf("unexpected output: got:\n%s\nwant\n%s\n", output, expected)
 	}
 }
+
+func TestDiagnostic_emptyOverlapHighlightContext(t *testing.T) {
+	var diags tfdiags.Diagnostics
+
+	diags = diags.Append(&hcl.Diagnostic{
+		Severity: hcl.DiagError,
+		Summary:  "Some error",
+		Detail:   "...",
+		Subject: &hcl.Range{
+			Filename: "source.tf",
+			Start:    hcl.Pos{Line: 3, Column: 10, Byte: 38},
+			End:      hcl.Pos{Line: 4, Column: 1, Byte: 39},
+		},
+		Context: &hcl.Range{
+			Filename: "source.tf",
+			Start:    hcl.Pos{Line: 2, Column: 13, Byte: 27},
+			End:      hcl.Pos{Line: 4, Column: 1, Byte: 39},
+		},
+	})
+	sources := map[string][]byte{
+		"source.tf": []byte(`variable "x" {
+  default = {
+    "foo"
+  }
+`),
+	}
+	color := &colorstring.Colorize{
+		Colors:  colorstring.DefaultColors,
+		Reset:   true,
+		Disable: true,
+	}
+	expected := `
+Error: Some error
+
+  on source.tf line 3, in variable "x":
+   2:   default = {
+   3:     "foo"
+   4:   }
+
+...
+`
+	output := Diagnostic(diags[0], sources, color, 80)
+
+	if output != expected {
+		t.Fatalf("unexpected output: got:\n%s\nwant\n%s\n", output, expected)
+	}
+}