patch 8.2.3658: duplicate code in xxd

Problem:    Duplicate code in xxd.
Solution:   Merge duplicated code. Add more tests. (closes #9192)
diff --git a/src/testdir/test_xxd.vim b/src/testdir/test_xxd.vim
index 7768f90..35d685f 100644
--- a/src/testdir/test_xxd.vim
+++ b/src/testdir/test_xxd.vim
@@ -263,6 +263,20 @@
   endfor
 endfunc
 
+func Test_xxd_ignore_garbage()
+  new
+  exe 'r! printf "\n\r xxxx 0: 42 42" | ' . s:xxd_cmd . ' -r'
+  call assert_match('BB', join(getline(1, 3)))
+  bwipe!
+endfunc
+
+func Test_xxd_bit_dump()
+  new
+  exe 'r! printf "123456" | ' . s:xxd_cmd . ' -b1'
+  call assert_match('00000000: 00110001 00110010 00110011 00110100 00110101 00110110  123456', join(getline(1, 3)))
+  bwipe!
+endfunc
+
 func Test_xxd_version()
   new
   exe 'r! ' . s:xxd_cmd . ' -v'
diff --git a/src/version.c b/src/version.c
index baca0af..a261ebd 100644
--- a/src/version.c
+++ b/src/version.c
@@ -758,6 +758,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3658,
+/**/
     3657,
 /**/
     3656,
diff --git a/src/xxd/xxd.c b/src/xxd/xxd.c
index cef3c61..b6756aa 100644
--- a/src/xxd/xxd.c
+++ b/src/xxd/xxd.c
@@ -809,33 +809,34 @@
   e = 0;
   while ((length < 0 || n < length) && (e = getc(fp)) != EOF)
     {
+      int x;
+
       if (p == 0)
 	{
 	  addrlen = sprintf(l, decimal_offset ? "%08ld:" : "%08lx:",
 				  ((unsigned long)(n + seekoff + displayoff)));
 	  for (c = addrlen; c < LLEN; l[c++] = ' ');
 	}
+      x = hextype == HEX_LITTLEENDIAN ? p ^ (octspergrp-1) : p;
+      c = addrlen + 1 + (grplen * x) / octspergrp;
       if (hextype == HEX_NORMAL || hextype == HEX_LITTLEENDIAN)
 	{
-	  int x = hextype == HEX_NORMAL ? p : p ^ (octspergrp-1);
-	  l[c = (addrlen + 1 + (grplen * x) / octspergrp)]
-	         = hexx[(e >> 4) & 0xf];
+	  l[c]   = hexx[(e >> 4) & 0xf];
 	  l[++c] = hexx[e & 0xf];
 	}
       else /* hextype == HEX_BITS */
 	{
 	  int i;
-
-	  c = (addrlen + 1 + (grplen * p) / octspergrp) - 1;
 	  for (i = 7; i >= 0; i--)
-	    l[++c] = (e & (1 << i)) ? '1' : '0';
+	    l[c++] = (e & (1 << i)) ? '1' : '0';
 	}
       if (e)
 	nonzero++;
       if (ebcdic)
 	e = (e < 64) ? '.' : etoa64[e-64];
       /* When changing this update definition of LLEN above. */
-      l[addrlen + 3 + (grplen * cols - 1)/octspergrp + p] =
+      c = addrlen + 3 + (grplen * cols - 1)/octspergrp + p;
+      l[c++] =
 #ifdef __MVS__
 	  (e >= 64)
 #else
@@ -845,7 +846,8 @@
       n++;
       if (++p == cols)
 	{
-	  l[c = (addrlen + 3 + (grplen * cols - 1)/octspergrp + p)] = '\n'; l[++c] = '\0';
+	  l[c] = '\n';
+	  l[++c] = '\0';
 	  xxdline(fpo, l, autoskip ? nonzero : 1);
 	  nonzero = 0;
 	  p = 0;
@@ -855,7 +857,8 @@
     perror_exit(2);
   if (p)
     {
-      l[c = (addrlen + 3 + (grplen * cols - 1)/octspergrp + p)] = '\n'; l[++c] = '\0';
+      l[c] = '\n';
+      l[++c] = '\0';
       xxdline(fpo, l, 1);
     }
   else if (autoskip)