apply: fix uninitialized hash function

"git apply" can work outside a repository as a better "GNU patch",
but when it does so, it still assumed that it can access
the_hash_algo, which is no longer true in the new world order.

Make sure we explicitly fall back to SHA-1 algorithm for backward
compatibility.

It is of dubious value to make this configurable to other hash
algorithms, as the code does not use the_hash_algo for hashing
purposes when working outside a repository (which is how
the_hash_algo is left to NULL)---it is only used to learn the max
length of the hash when parsing the object names on the "index"
line, but failing to parse the "index" line is not a hard failure,
and the program does not support operations like applying binary
patches and --3way fallback that requires object access outside a
repository.

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

builtin/apply.c

@@ -1,6 +1,7 @@
 #include "builtin.h"
 #include "gettext.h"
 #include "repository.h"
+#include "hash.h"
 #include "apply.h"
 
 static const char * const apply_usage[] = {
@@ -18,6 +19,15 @@ int cmd_apply(int argc, const char **argv, const char *prefix)
 	if (init_apply_state(&state, the_repository, prefix))
 		exit(128);
 
+	/*
+	 * We could to redo the "apply.c" machinery to make this
+	 * arbitrary fallback unnecessary, but it is dubious that it
+	 * is worth the effort.
+	 * cf. https://lore.kernel.org/git/xmqqcypfcmn4.fsf@gitster.g/
+	 */
+	if (!the_hash_algo)
+		repo_set_hash_algo(the_repository, GIT_HASH_SHA1);
+
 	argc = apply_parse_options(argc, argv,
 				   &state, &force_apply, &options,
 				   apply_usage);

t/t1517-outside-repo.sh

@@ -37,7 +37,7 @@ test_expect_success 'hash-object outside repository (uses SHA-1)' '
 	test_cmp hash.expect hash.actual
 '
 
-test_expect_failure 'apply a patch outside repository' '
+test_expect_success 'apply a patch outside repository' '
 	(
 		cd non-repo &&
 		cp ../nums.old nums &&