Merge branch 'bc/sha-256-part-1-of-4'

SHA-256 transition continues.

* bc/sha-256-part-1-of-4: (22 commits)
  fast-import: add options for rewriting submodules
  fast-import: add a generic function to iterate over marks
  fast-import: make find_marks work on any mark set
  fast-import: add helper function for inserting mark object entries
  fast-import: permit reading multiple marks files
  commit: use expected signature header for SHA-256
  worktree: allow repository version 1
  init-db: move writing repo version into a function
  builtin/init-db: add environment variable for new repo hash
  builtin/init-db: allow specifying hash algorithm on command line
  setup: allow check_repository_format to read repository format
  t/helper: make repository tests hash independent
  t/helper: initialize repository if necessary
  t/helper/test-dump-split-index: initialize git repository
  t6300: make hash algorithm independent
  t6300: abstract away SHA-1-specific constants
  t: use hash-specific lookup tables to define test constants
  repository: require a build flag to use SHA-256
  hex: add functions to parse hex object IDs in any algorithm
  hex: introduce parsing variants taking hash algorithms
  ...

builtin/clone.c

@@ -1106,7 +1106,7 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 		}
 	}
 
-	init_db(git_dir, real_git_dir, option_template, INIT_DB_QUIET);
+	init_db(git_dir, real_git_dir, option_template, GIT_HASH_UNKNOWN, INIT_DB_QUIET);
 
 	if (real_git_dir)
 		git_dir = real_git_dir;