refactor get_textconv to not require diff_filespec

This function actually does two things:

  1. Load the userdiff driver for the filespec.

  2. Decide whether the driver has a textconv component, and
     initialize the textconv cache if applicable.

Only part (1) requires the filespec object, and some callers
may not have a filespec at all. So let's split them it into
two functions, and put part (2) with the userdiff code,
which is a better fit.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -1976,19 +1976,7 @@ struct userdiff_driver *get_textconv(struct diff_filespec *one)
 		return NULL;
 
 	diff_filespec_load_driver(one);
-	if (!one->driver->textconv)
-		return NULL;
-
-	if (one->driver->textconv_want_cache && !one->driver->textconv_cache) {
-		struct notes_cache *c = xmalloc(sizeof(*c));
-		struct strbuf name = STRBUF_INIT;
-
-		strbuf_addf(&name, "textconv/%s", one->driver->name);
-		notes_cache_init(c, name.buf, one->driver->textconv);
-		one->driver->textconv_cache = c;
-	}
-
-	return one->driver;
+	return userdiff_get_textconv(one->driver);
 }
 
 static void builtin_diff(const char *name_a,

userdiff.c

@@ -267,3 +267,20 @@ struct userdiff_driver *userdiff_find_by_path(const char *path)
 		return NULL;
 	return userdiff_find_by_name(check.value);
 }
+
+struct userdiff_driver *userdiff_get_textconv(struct userdiff_driver *driver)
+{
+	if (!driver->textconv)
+		return NULL;
+
+	if (driver->textconv_want_cache && !driver->textconv_cache) {
+		struct notes_cache *c = xmalloc(sizeof(*c));
+		struct strbuf name = STRBUF_INIT;
+
+		strbuf_addf(&name, "textconv/%s", driver->name);
+		notes_cache_init(c, name.buf, driver->textconv);
+		driver->textconv_cache = c;
+	}
+
+	return driver;
+}

userdiff.h

@@ -23,4 +23,6 @@ int userdiff_config(const char *k, const char *v);
 struct userdiff_driver *userdiff_find_by_name(const char *name);
 struct userdiff_driver *userdiff_find_by_path(const char *path);
 
+struct userdiff_driver *userdiff_get_textconv(struct userdiff_driver *driver);
+
 #endif /* USERDIFF */