tests: use test_ln_s_add to remove SYMLINKS prerequisite (trivial cases)

There are many instances where the treatment of symbolic links in the
object model and the algorithms are tested, but where it is not
necessary to actually have a symbolic link in the worktree. Make
adjustments to the tests and remove the SYMLINKS prerequisite when
appropriate in trivial cases, where "trivial" means:

- merely a replacement of 'ln -s a b && git add b' by test_ln_s_add
  is needed;

- a test for symbolic link on the file system can be split off (and
  remains protected by SYMLINKS);

- existing code is equivalent to test_ln_s_add.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9500-gitweb-standalone-no-errors.sh

@@ -156,10 +156,10 @@ test_expect_success \
 	 git commit -a -m "File renamed." &&
 	 gitweb_run "p=.git;a=commitdiff"'
 
-test_expect_success SYMLINKS \
+test_expect_success \
 	'commitdiff(0): file to symlink' \
 	'rm renamed_file &&
-	 ln -s file renamed_file &&
+	 test_ln_s_add file renamed_file &&
 	 git commit -a -m "File to symlink." &&
 	 gitweb_run "p=.git;a=commitdiff"'
 
@@ -212,15 +212,14 @@ test_expect_success \
 # ----------------------------------------------------------------------
 # commitdiff testing (taken from t4114-apply-typechange.sh)
 
-test_expect_success SYMLINKS 'setup typechange commits' '
+test_expect_success 'setup typechange commits' '
 	echo "hello world" > foo &&
 	echo "hi planet" > bar &&
 	git update-index --add foo bar &&
 	git commit -m initial &&
 	git branch initial &&
 	rm -f foo &&
-	ln -s bar foo &&
-	git update-index foo &&
+	test_ln_s_add bar foo &&
 	git commit -m "foo symlinked to bar" &&
 	git branch foo-symlinked-to-bar &&
 	rm -f foo &&
@@ -361,11 +360,7 @@ test_expect_success \
 	 echo "Changed" >> 04-rename-to &&
 	 test_chmod +x 05-mode-change &&
 	 rm -f 06-file-or-symlink &&
-	 if test_have_prereq SYMLINKS; then
-		ln -s 01-change 06-file-or-symlink
-	 else
-		printf %s 01-change > 06-file-or-symlink
-	 fi &&
+	 test_ln_s_add 01-change 06-file-or-symlink &&
 	 echo "Changed and have mode changed" > 07-change-mode-change	&&
 	 test_chmod +x 07-change-mode-change &&
 	 git commit -a -m "Large commit" &&