check_path(): allow symlinked directories to checkout-index --prefix

Merlyn noticed that Documentation/install-doc-quick.sh no longer correctly
removes old installed documents when the target directory has a leading
path that is a symlink.  It turns out that "checkout-index --prefix" was
broken by recent b6986d8 (git-checkout: be careful about untracked
symlinks, 2009-07-29).

I suspect has_symlink_leading_path() could learn the third parameter
(prefix that is allowed to be symlinked directories) to allow us to retire
a similar function has_dirs_only_path().

Another avenue of fixing this I considered was to get rid of base_dir and
base_dir_len from "struct checkout", and instead make "git checkout-index"
when run with --prefix mkdir the leading path and chdir in there.  It
might be the best longer term solution to this issue, as the base_dir
feature is used only by that rather obscure codepath as far as I know.

But at least this patch should fix this breakage.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2000-checkout-cache-clash.sh

@@ -48,4 +48,13 @@ test_expect_success \
     'git checkout-index conflicting paths.' \
     'test -f path0 && test -d path1 && test -f path1/file1'
 
+test_expect_success SYMLINKS 'checkout-index -f twice with --prefix' '
+	mkdir -p tar/get &&
+	ln -s tar/get there &&
+	echo first &&
+	git checkout-index -a -f --prefix=there/ &&
+	echo second &&
+	git checkout-index -a -f --prefix=there/
+'
+
 test_done