patch 8.2.3468: problem with :cd when editing file in non-existent directory

Problem:    Problem with :cd when editing file in non-existent directory. (Yee
            Cheng Chin)
Solution:   Prepend the current directory to get the full path. (closes #8903)

src/os_unix.c

@@ -2647,7 +2647,15 @@ mch_FullName(
 		{
 		    vim_strncpy(buf, fname, p - fname);
 		    if (mch_chdir((char *)buf))
+		    {
+			// Path does not exist (yet).  For a full path fail,
+			// will use the path as-is.  For a relative path use
+			// the current directory and append the file name.
+			if (mch_isFullName(fname))
 			    retval = FAIL;
+			else
+			    p = NULL;
+		    }
 		    else if (*p == '/')
 			fname = p + 1;
 		    else

src/testdir/test_cd.vim

@@ -208,4 +208,21 @@ func Test_cd_completion()
   call delete('XComplFile')
 endfunc
 
+func Test_cd_unknown_dir()
+  call mkdir('Xa')
+  cd Xa
+  call writefile(['text'], 'Xb.txt')
+  edit Xa/Xb.txt
+  let first_buf = bufnr()
+  cd ..
+  edit
+  call assert_equal(first_buf, bufnr())
+  edit Xa/Xb.txt
+  call assert_notequal(first_buf, bufnr())
+
+  bwipe!
+  exe "bwipe! " .. first_buf
+  call delete('Xa', 'rf')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3468,
 /**/
     3467,
 /**/