cache-tree tests: explicitly test HEAD and index differences

cache-tree tests: explicitly test HEAD and index differences

The test code added in 9c4d6c0297 (cache-tree: Write updated
cache-tree after commit, 2014-07-13) used "ls-files" in lieu of
"ls-tree" because it wanted to test the data in the index, since this
test is testing the cache-tree extension.

Change the test to instead use "ls-tree" for traversal, and then
explicitly check how HEAD differs from the index. This is more easily
understood, and less fragile as numerous past bug fixes[1][2][3] to
the old code we're replacing demonstrate.

As an aside this would be a bit easier if empty pathspecs hadn't been
made an error in d426430e6e (pathspec: warn on empty strings as
pathspec, 2016-06-22) and 9e4e8a64c2 (pathspec: die on empty strings
as pathspec, 2017-06-06).

If that was still allowed this code could be simplified slightly:

	diff --git a/t/t0090-cache-tree.sh b/t/t0090-cache-tree.sh
	index 9bf66c9e68..0b02881f55 100755
	--- a/t/t0090-cache-tree.sh
	+++ b/t/t0090-cache-tree.sh
	@@ -18,19 +18,18 @@ cmp_cache_tree () {
	 # test-tool dump-cache-tree already verifies that all existing data is
	 # correct.
	 generate_expected_cache_tree () {
	-       pathspec="$1" &&
	-       dir="$2${2:+/}" &&
	+       pathspec="$1${1:+/}" &&
	        git ls-tree --name-only HEAD -- "$pathspec" >files &&
	        git ls-tree --name-only -d HEAD -- "$pathspec" >subtrees &&
	-       printf "SHA %s (%d entries, %d subtrees)\n" "$dir" $(wc -l <files) $(wc -l <subtrees) &&
	+       printf "SHA %s (%d entries, %d subtrees)\n" "$pathspec" $(wc -l <files) $(wc -l <subtrees) &&
	        while read subtree
	        do
	-               generate_expected_cache_tree "$pathspec/$subtree/" "$subtree" || return 1
	+               generate_expected_cache_tree "$subtree" || return 1
	        done <subtrees
	 }

	 test_cache_tree () {
	-       generate_expected_cache_tree "." >expect &&
	+       generate_expected_cache_tree >expect &&
	        cmp_cache_tree expect &&
	        rm expect actual files subtrees &&
	        git status --porcelain -- ':!status' ':!expected.status' >status &&

1. c8db708d5d (t0090: avoid passing empty string to printf %d,
   2014-09-30)
2. d69360c6b1 (t0090: tweak awk statement for Solaris
   /usr/xpg4/bin/awk, 2014-12-22)
3. 9b5a9fa60a (t0090: stop losing return codes of git commands,
   2019-11-27)

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>