git add --intent-to-add: fix removal of cached emptiness

This uses the extended index flag mechanism introduced earlier to mark
the entries added to the index via "git add -N" with CE_INTENT_TO_ADD.

The logic to detect an "intent to add" entry for the purpose of allowing
"git rm --cached $path" is tightened to check not just for a staged empty
blob, but with the CE_INTENT_TO_ADD bit.  This protects an empty blob that
was explicitly added and then modified in the work tree from being dropped
with this sequence:

	$ >empty
	$ git add empty
	$ echo "non empty" >empty
	$ git rm --cached empty

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

builtin-rm.c

@@ -115,7 +115,7 @@ static int check_local_mod(unsigned char *head, int index_only)
 		 * "intent to add" entry.
 		 */
 		if (local_changes && staged_changes) {
-			if (!index_only || !is_empty_blob_sha1(ce->sha1))
+			if (!index_only || !(ce->ce_flags & CE_INTENT_TO_ADD))
 				errs = error("'%s' has staged content different "
 					     "from both the file and the HEAD\n"
 					     "(use -f to force removal)", name);

cache.h

@@ -176,10 +176,11 @@ struct cache_entry {
 /*
  * Extended on-disk flags
  */
+#define CE_INTENT_TO_ADD 0x20000000
 /* CE_EXTENDED2 is for future extension */
 #define CE_EXTENDED2 0x80000000
 
-#define CE_EXTENDED_FLAGS (0)
+#define CE_EXTENDED_FLAGS (CE_INTENT_TO_ADD)
 
 /*
  * Safeguard to avoid saving wrong flags:

read-cache.c

@@ -546,6 +546,8 @@ int add_to_index(struct index_state *istate, const char *path, struct stat *st,
 	ce->ce_flags = namelen;
 	if (!intent_only)
 		fill_stat_cache_info(ce, st);
+	else
+		ce->ce_flags |= CE_INTENT_TO_ADD;
 
 	if (trust_executable_bit && has_symlinks)
 		ce->ce_mode = create_ce_mode(st_mode);

t/t3600-rm.sh

@@ -187,8 +187,8 @@ test_expect_success 'but with -f it should work.' '
 	test_must_fail git ls-files --error-unmatch baz
 '
 
-test_expect_failure 'refuse to remove cached empty file with modifications' '
-	touch empty &&
+test_expect_success 'refuse to remove cached empty file with modifications' '
+	>empty &&
 	git add empty &&
 	echo content >empty &&
 	test_must_fail git rm --cached empty