Teach notes code to free its internal data structures on request

There's no need to be rude to memory-concious callers...

This patch has been improved by the following contributions:
- Junio C Hamano: avoid old-style declaration

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

notes.c

@@ -105,6 +105,13 @@ static unsigned char *lookup_notes(const unsigned char *commit_sha1)
 	return hash_map.entries[index].notes_sha1;
 }
 
+void free_notes(void)
+{
+	free(hash_map.entries);
+	memset(&hash_map, 0, sizeof(struct hash_map));
+	initialized = 0;
+}
+
 void get_commit_notes(const struct commit *commit, struct strbuf *sb,
 		const char *output_encoding, int flags)
 {

notes.h

@@ -1,6 +1,9 @@
 #ifndef NOTES_H
 #define NOTES_H
 
+/* Free (and de-initialize) the internal notes tree structure */
+void free_notes(void);
+
 #define NOTES_SHOW_HEADER 1
 #define NOTES_INDENT 2