strbuf: separate callback for strbuf_expand:ing literals

Expanding '%n' and '%xNN' is generic functionality, so extract that from
the pretty.c formatter into a callback that can be reused.

No functional change intended

Signed-off-by: Anders Waldenborg <anders@0x63.nu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pretty.c

@@ -1137,9 +1137,13 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 	const char *msg = c->message;
 	struct commit_list *p;
 	const char *arg;
-	int ch;
+	size_t res;
 
 	/* these are independent of the commit */
+	res = strbuf_expand_literal_cb(sb, placeholder, NULL);
+	if (res)
+		return res;
+
 	switch (placeholder[0]) {
 	case 'C':
 		if (starts_with(placeholder + 1, "(auto)")) {
@@ -1158,16 +1162,6 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			 */
 			return ret;
 		}
-	case 'n':		/* newline */
-		strbuf_addch(sb, '\n');
-		return 1;
-	case 'x':
-		/* %x00 == NUL, %x0a == LF, etc. */
-		ch = hex2chr(placeholder + 1);
-		if (ch < 0)
-			return 0;
-		strbuf_addch(sb, ch);
-		return 3;
 	case 'w':
 		if (placeholder[1] == '(') {
 			unsigned long width = 0, indent1 = 0, indent2 = 0;

strbuf.c

@@ -380,6 +380,27 @@ void strbuf_expand(struct strbuf *sb, const char *format, expand_fn_t fn,
 	}
 }
 
+size_t strbuf_expand_literal_cb(struct strbuf *sb,
+				const char *placeholder,
+				void *context)
+{
+	int ch;
+
+	switch (placeholder[0]) {
+	case 'n':		/* newline */
+		strbuf_addch(sb, '\n');
+		return 1;
+	case 'x':
+		/* %x00 == NUL, %x0a == LF, etc. */
+		ch = hex2chr(placeholder + 1);
+		if (ch < 0)
+			return 0;
+		strbuf_addch(sb, ch);
+		return 3;
+	}
+	return 0;
+}
+
 size_t strbuf_expand_dict_cb(struct strbuf *sb, const char *placeholder,
 		void *context)
 {

strbuf.h

@@ -320,6 +320,14 @@ void strbuf_expand(struct strbuf *sb,
 		   expand_fn_t fn,
 		   void *context);
 
+/**
+ * Used as callback for `strbuf_expand` to only expand literals
+ * (i.e. %n and %xNN). The context argument is ignored.
+ */
+size_t strbuf_expand_literal_cb(struct strbuf *sb,
+				const char *placeholder,
+				void *context);
+
 /**
  * Used as callback for `strbuf_expand()`, expects an array of
  * struct strbuf_expand_dict_entry as context, i.e. pairs of