Merge branch 'nd/sparse'

* nd/sparse: (25 commits)
  t7002: test for not using external grep on skip-worktree paths
  t7002: set test prerequisite "external-grep" if supported
  grep: do not do external grep on skip-worktree entries
  commit: correctly respect skip-worktree bit
  ie_match_stat(): do not ignore skip-worktree bit with CE_MATCH_IGNORE_VALID
  tests: rename duplicate t1009
  sparse checkout: inhibit empty worktree
  Add tests for sparse checkout
  read-tree: add --no-sparse-checkout to disable sparse checkout support
  unpack-trees(): ignore worktree check outside checkout area
  unpack_trees(): apply $GIT_DIR/info/sparse-checkout to the final index
  unpack-trees(): "enable" sparse checkout and load $GIT_DIR/info/sparse-checkout
  unpack-trees.c: generalize verify_* functions
  unpack-trees(): add CE_WT_REMOVE to remove on worktree alone
  Introduce "sparse checkout"
  dir.c: export excluded_1() and add_excludes_from_file_1()
  excluded_1(): support exclude files in index
  unpack-trees(): carry skip-worktree bit over in merged_entry()
  Read .gitignore from index if it is skip-worktree
  Avoid writing to buffer in add_excludes_from_file_1()
  ...

Conflicts:
	.gitignore
	Documentation/config.txt
	Documentation/git-update-index.txt
	Makefile
	entry.c
	t/t7002-grep.sh

config.c

@@ -518,6 +518,11 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.sparsecheckout")) {
+		core_apply_sparse_checkout = git_config_bool(var, value);
+		return 0;
+	}
+
 	/* Add other config variables here and to Documentation/config.txt. */
 	return 0;
 }