patch 8.2.3388: fnamemodify('path/..', ':p') differs from using 'path/../'

Problem:    fnamemodify('path/..', ':p') differs from using 'path/../'.
Solution:   Include the "/.." in the directory name. (closes #8808)

Author: brammool

src/os_unix.c

@@ -2602,6 +2602,10 @@ mch_FullName(
 	 */
 	if (p != NULL)
 	{
+	    if (STRCMP(p, "/..") == 0)
+		// for "/path/dir/.." include the "/.."
+		p += 3;
+
 #ifdef HAVE_FCHDIR
 	    /*
 	     * Use fchdir() if possible, it's said to be faster and more
@@ -2644,8 +2648,10 @@ mch_FullName(
 		    vim_strncpy(buf, fname, p - fname);
 		    if (mch_chdir((char *)buf))
 			retval = FAIL;
-		    else
+		    else if (*p == '/')
 			fname = p + 1;
+		    else
+			fname = p;
 		    *buf = NUL;
 		}
 	    }

src/testdir/test_fnamemodify.vim

@@ -12,6 +12,8 @@ func Test_fnamemodify()
   call assert_equal('r', fnamemodify('.', ':p:h')[-1:])
   call assert_equal('t', fnamemodify('test.out', ':p')[-1:])
   call assert_equal($HOME .. "/foo" , fnamemodify('~/foo', ':p'))
+  call assert_equal(fnamemodify('.', ':p:h:h:h') .. '/', fnamemodify($HOME .. '/../', ':p'))
+  call assert_equal(fnamemodify('.', ':p:h:h:h') .. '/', fnamemodify($HOME .. '/..', ':p'))
   call assert_equal('test.out', fnamemodify('test.out', ':.'))
   call assert_equal('a', fnamemodify('../testdir/a', ':.'))
   call assert_equal('~/testdir/test.out', fnamemodify('test.out', ':~'))

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3388,
 /**/
     3387,
 /**/