pull: support pull.ff config

Since b814da8 (pull: add pull.ff configuration, 2014-01-15), git-pull.sh
would lookup the configuration value of "pull.ff", and set the flag
"--ff" if its value is "true", "--no-ff" if its value is "false" and
"--ff-only" if its value is "only".

Re-implement this behavior.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pull.c

@@ -166,6 +166,32 @@ static void argv_push_force(struct argv_array *arr)
 		argv_array_push(arr, "-f");
 }
 
+/**
+ * If pull.ff is unset, returns NULL. If pull.ff is "true", returns "--ff". If
+ * pull.ff is "false", returns "--no-ff". If pull.ff is "only", returns
+ * "--ff-only". Otherwise, if pull.ff is set to an invalid value, die with an
+ * error.
+ */
+static const char *config_get_ff(void)
+{
+	const char *value;
+
+	if (git_config_get_value("pull.ff", &value))
+		return NULL;
+
+	switch (git_config_maybe_bool("pull.ff", value)) {
+	case 0:
+		return "--no-ff";
+	case 1:
+		return "--ff";
+	}
+
+	if (!strcmp(value, "only"))
+		return "--ff-only";
+
+	die(_("Invalid value for pull.ff: %s"), value);
+}
+
 /**
  * Appends merge candidates from FETCH_HEAD that are not marked not-for-merge
  * into merge_heads.
@@ -397,6 +423,9 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 
 	parse_repo_refspecs(argc, argv, &repo, &refspecs);
 
+	if (!opt_ff)
+		opt_ff = xstrdup_or_null(config_get_ff());
+
 	if (run_fetch(repo, refspecs))
 		return 1;