diff: don’t backspace before first column

* src/util.c (output_1_line): When expanding tabs, treat backspace
before column 1 as no-op, since that’s what most devices do.
* tests/expand-tabs: New test.
* tests/Makefile.am (TESTS): Add it.

src/util.c

@@ -1397,12 +1397,15 @@ output_1_line (char const *base, char const *limit, char const *flag_format,
               break;
 
             case '\b':
-              column--;
-	      if (column < 0)
+	      if (0 < column)
+		column--;
+	      else if (0 < tab)
 		{
 		  tab--;
 		  column = tab_size - 1;
 		}
+	      else
+		continue;
               putc (c, out);
               break;
 

tests/Makefile.am

@@ -10,6 +10,7 @@ TESTS = \
   colliding-file-names \
   diff3 \
   excess-slash \
+  expand-tabs \
   help-version	\
   ifdef \
   invalid-re	\

tests/expand-tabs

@@ -0,0 +1,19 @@
+#!/bin/sh
+# Test tab expansion.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+
+fail=0
+
+cat >exp <<'EOF' || framework_failure_
+0a1
+>         x
+EOF
+
+for p in '\b\tx\n' '\b        x\n' '\b \tx\n'; do
+  printf "$p" | returns_ 1 diff -t /dev/null - >out || fail=1
+done
+
+compare exp out || fail=1
+
+Exit $fail