grep: don't emit the -T alignment tab on empty lines

With -T, grep pads the prefix with a tab so line content lands on a tab
stop. GNU omits that tab when the line has no content: an empty line
prints just its prefix (a whitespace-only line still gets the tab).
uu_grep always wrote the tab, so empty matched lines gained a spurious
trailing tab. Gate the tab on non-empty content. Fixes the GNU testsuite
'initial-tab' test.

src/output.rs

@@ -71,6 +71,7 @@ impl<'a> OutputWriter<'a> {
                 view.line_number,
                 view.byte_offset + start as u64,
                 b':',
+                false,
             )?;
 
             self.write_colored_bytes(
@@ -90,6 +91,7 @@ impl<'a> OutputWriter<'a> {
             view.line_number,
             view.byte_offset,
             if view.is_match { b':' } else { b'-' },
+            view.line.is_empty(),
         )?;
 
         let mut last_end = 0;
@@ -125,6 +127,7 @@ impl<'a> OutputWriter<'a> {
         line_number: u64,
         byte_offset: u64,
         sep_char: u8,
+        content_empty: bool,
     ) -> io::Result<()> {
         if self.config.show_filename {
             self.write_colored_fmt(
@@ -155,7 +158,10 @@ impl<'a> OutputWriter<'a> {
             self.write_separator(sep_char)?;
         }
 
+        // GNU grep aligns content with a tab under -T, but only when there is
+        // content to align: an empty line keeps just its prefix (no tab).
         if self.config.initial_tab
+            && !content_empty
             && (self.config.line_number || self.config.byte_offset || self.config.show_filename)
         {
             self.out.write_all(b"\t")?;

tests/test_grep.rs

@@ -126,6 +126,24 @@ fn ere_invalid_pattern_is_error() {
         .stderr_contains("invalid pattern");
 }
 
+#[test]
+fn initial_tab_skips_empty_lines() {
+    // -T aligns content with a tab, but GNU omits the tab for an empty line
+    // (a whitespace-only line still gets one). -H forces the filename prefix
+    // on, so the tab is exercised.
+    let (s, mut c) = ucmd();
+    s.fixtures.write("in", "x\n\n");
+    c.args(&["-T", "-H", "^", "in"])
+        .succeeds()
+        .stdout_is("in:\tx\nin:\n");
+
+    let (s, mut c) = ucmd();
+    s.fixtures.write("in", "x\n \n");
+    c.args(&["-T", "-H", "^", "in"])
+        .succeeds()
+        .stdout_is("in:\tx\nin:\t \n");
+}
+
 #[test]
 fn fixed_string_is_literal() {
     // Metacharacters are not interpreted.