Add new git_signature_default API using config

This adds a new API for creating a signature that uses the
config to look up "user.name" and "user.email".

include/git2/signature.h

@@ -48,6 +48,19 @@ GIT_EXTERN(int) git_signature_new(git_signature **out, const char *name, const c
  */
 GIT_EXTERN(int) git_signature_now(git_signature **out, const char *name, const char *email);
 
+/**
+ * Create a new action signature with default user and now timestamp.
+ *
+ * This looks up the user.name and user.email from the configuration and
+ * uses the current time as the timestamp, and creates a new signature
+ * based on that information.  It will return GIT_ENOTFOUND if either the
+ * user.name or user.email are not set.
+ *
+ * @param out new signature
+ * @param repo repository pointer
+ * @return 0 on success, GIT_ENOTFOUND if config is missing, or error code
+ */
+GIT_EXTERN(int) git_signature_default(git_signature **out, git_repository *repo);
 
 /**
  * Create a copy of an existing signature.  All internal strings are also

src/signature.c

@@ -129,6 +129,23 @@ int git_signature_now(git_signature **sig_out, const char *name, const char *ema
 	return 0;
 }
 
+int git_signature_default(git_signature **out, git_repository *repo)
+{
+	int error;
+	git_config *cfg;
+	const char *user_name, *user_email;
+
+	if ((error = git_repository_config(&cfg, repo)) < 0)
+		return error;
+
+	if (!(error = git_config_get_string(&user_name, cfg, "user.name")) &&
+		!(error = git_config_get_string(&user_email, cfg, "user.email")))
+		error = git_signature_now(out, user_name, user_email);
+
+	git_config_free(cfg);
+	return error;
+}
+
 int git_signature__parse(git_signature *sig, const char **buffer_out,
 		const char *buffer_end, const char *header, char ender)
 {