Merge branch 'ps/undecided-is-not-necessarily-sha1'

Before discovering the repository details, We used to assume SHA-1
as the "default" hash function, which has been corrected. Hopefully
this will smoke out codepaths that rely on such an unwarranted
assumptions.

* ps/undecided-is-not-necessarily-sha1:
  repository: stop setting SHA1 as the default object hash
  oss-fuzz/commit-graph: set up hash algorithm
  builtin/shortlog: don't set up revisions without repo
  builtin/diff: explicitly set hash algo when there is no repo
  builtin/bundle: abort "verify" early when there is no repository
  builtin/blame: don't access potentially unitialized `the_hash_algo`
  builtin/rev-parse: allow shortening to more than 40 hex characters
  remote-curl: fix parsing of detached SHA256 heads
  attr: fix BUG() when parsing attrs outside of repo
  attr: don't recompute default attribute source
  parse-options-cb: only abbreviate hashes when hash algo is known
  path: move `validate_headref()` to its only user
  path: harden validation of HEAD with non-standard hashes

setup.c

@@ -4,6 +4,7 @@
 #include "environment.h"
 #include "exec-cmd.h"
 #include "gettext.h"
+#include "hex.h"
 #include "object-name.h"
 #include "refs.h"
 #include "repository.h"
@@ -342,6 +343,58 @@ int get_common_dir_noenv(struct strbuf *sb, const char *gitdir)
 	return ret;
 }
 
+static int validate_headref(const char *path)
+{
+	struct stat st;
+	char buffer[256];
+	const char *refname;
+	struct object_id oid;
+	int fd;
+	ssize_t len;
+
+	if (lstat(path, &st) < 0)
+		return -1;
+
+	/* Make sure it is a "refs/.." symlink */
+	if (S_ISLNK(st.st_mode)) {
+		len = readlink(path, buffer, sizeof(buffer)-1);
+		if (len >= 5 && !memcmp("refs/", buffer, 5))
+			return 0;
+		return -1;
+	}
+
+	/*
+	 * Anything else, just open it and try to see if it is a symbolic ref.
+	 */
+	fd = open(path, O_RDONLY);
+	if (fd < 0)
+		return -1;
+	len = read_in_full(fd, buffer, sizeof(buffer)-1);
+	close(fd);
+
+	if (len < 0)
+		return -1;
+	buffer[len] = '\0';
+
+	/*
+	 * Is it a symbolic ref?
+	 */
+	if (skip_prefix(buffer, "ref:", &refname)) {
+		while (isspace(*refname))
+			refname++;
+		if (starts_with(refname, "refs/"))
+			return 0;
+	}
+
+	/*
+	 * Is this a detached HEAD?
+	 */
+	if (get_oid_hex_any(buffer, &oid) != GIT_HASH_UNKNOWN)
+		return 0;
+
+	return -1;
+}
+
 /*
  * Test if it looks like we're at a git directory.
  * We want to see: