Major submodule rewrite

This replaces the old submodule API with a new extended API that
supports most of the things that can be done with `git submodule`.

Author: arrbee

include/git2/errors.h

@@ -54,6 +54,7 @@ typedef enum {
 	GITERR_TREE,
 	GITERR_INDEXER,
 	GITERR_SSL,
+	GITERR_SUBMODULE,
 } git_error_t;
 
 /**

include/git2/submodule.h

@@ -253,11 +253,17 @@ static int config_set(git_config_file *cfg, const char *name, const char *value)
 		char *tmp = NULL;
 
 		git__free(key);
+
 		if (existing->next != NULL) {
 			giterr_set(GITERR_CONFIG, "Multivar incompatible with simple set");
 			return -1;
 		}
 
+		/* don't update if old and new values already match */
+		if ((!existing->value && !value) ||
+			(existing->value && value && !strcmp(existing->value, value)))
+			return 0;
+
 		if (value) {
 			tmp = git__strdup(value);
 			GITERR_CHECK_ALLOC(tmp);

src/config_file.c

@@ -19,12 +19,24 @@ GIT_INLINE(void) git_config_file_free(git_config_file *cfg)
 	cfg->free(cfg);
 }
 
+GIT_INLINE(int) git_config_file_get_string(
+	const char **out, git_config_file *cfg, const char *name)
+{
+	return cfg->get(cfg, name, out);
+}
+
 GIT_INLINE(int) git_config_file_set_string(
 	git_config_file *cfg, const char *name, const char *value)
 {
 	return cfg->set(cfg, name, value);
 }
 
+GIT_INLINE(int) git_config_file_delete(
+	git_config_file *cfg, const char *name)
+{
+	return cfg->del(cfg, name);
+}
+
 GIT_INLINE(int) git_config_file_foreach(
 	git_config_file *cfg,
 	int (*fn)(const char *key, const char *value, void *data),

src/config_file.h

@@ -530,7 +530,7 @@ static int maybe_modified(
 				status = GIT_DELTA_UNMODIFIED;
 			else if (git_submodule_lookup(&sub, diff->repo, nitem->path) < 0)
 				return -1;
-			else if (sub->ignore == GIT_SUBMODULE_IGNORE_ALL)
+			else if (git_submodule_ignore(sub) == GIT_SUBMODULE_IGNORE_ALL)
 				status = GIT_DELTA_UNMODIFIED;
 			else {
 				/* TODO: support other GIT_SUBMODULE_IGNORE values */