fold: variable width tabs, guard treating tab as whitespace

Treat tab chars as advancing to the next tab stop rather than having a fixed
8-column width.

Also treat tab as a whitespace split target only when splitting on word
boundaries.

tests/by-util/test_fold.rs

@@ -132,7 +132,7 @@ fn test_single_tab_should_not_add_extra_newline() {
 }
 
 #[test]
-fn test_tab_counts_as_8_columns() {
+fn test_initial_tab_counts_as_8_columns() {
     new_ucmd!()
         .arg("-w8")
         .pipe_in("\t1")
@@ -140,6 +140,33 @@ fn test_tab_counts_as_8_columns() {
         .stdout_is("\t\n1");
 }
 
+#[test]
+fn test_tab_should_advance_to_next_tab_stop() {
+    // tab advances the column count to the next tab stop, i.e. the width
+    // of the tab varies based on the leading text
+    new_ucmd!()
+        .args(&["-w8", "tab_stops.input"])
+        .succeeds()
+        .stdout_is_fixture("tab_stops_w8.expected");
+}
+
+#[test]
+fn test_all_tabs_should_advance_to_next_tab_stops() {
+    new_ucmd!()
+        .args(&["-w16", "tab_stops.input"])
+        .succeeds()
+        .stdout_is_fixture("tab_stops_w16.expected");
+}
+
+#[test]
+fn test_fold_before_tab_with_narrow_width() {
+    new_ucmd!()
+        .arg("-w7")
+        .pipe_in("a\t1")
+        .succeeds()
+        .stdout_is("a\n\t\n1");
+}
+
 #[test]
 fn test_fold_at_word_boundary() {
     new_ucmd!()
@@ -167,8 +194,35 @@ fn test_fold_at_word_boundary_preserve_final_newline() {
         .stdout_is("one \ntwo\n");
 }
 
+#[test]
+fn test_fold_at_tab() {
+    new_ucmd!()
+        .arg("-w8")
+        .pipe_in("a\tbbb\n")
+        .succeeds()
+        .stdout_is("a\t\nbbb\n");
+}
+
+#[test]
+fn test_fold_after_tab() {
+    new_ucmd!()
+        .arg("-w10")
+        .pipe_in("a\tbbb\n")
+        .succeeds()
+        .stdout_is("a\tbb\nb\n");
+}
+
 #[test]
 fn test_fold_at_tab_as_word_boundary() {
+    new_ucmd!()
+        .args(&["-w8", "-s"])
+        .pipe_in("a\tbbb\n")
+        .succeeds()
+        .stdout_is("a\t\nbbb\n");
+}
+
+#[test]
+fn test_fold_after_tab_as_word_boundary() {
     new_ucmd!()
         .args(&["-w10", "-s"])
         .pipe_in("a\tbbb\n")
@@ -317,6 +371,15 @@ fn test_tab_counts_as_one_byte() {
         .stdout_is("1\t\n2\n");
 }
 
+#[test]
+fn test_bytewise_fold_before_tab_with_narrow_width() {
+    new_ucmd!()
+        .args(&["-w7", "-b"])
+        .pipe_in("a\t1")
+        .succeeds()
+        .stdout_is("a\t1");
+}
+
 #[test]
 fn test_bytewise_fold_at_word_boundary_only_whitespace() {
     new_ucmd!()