hashmap: add string interning API

Interning short strings with high probability of duplicates can reduce the
memory footprint and speed up comparisons.

Add strintern() and memintern() APIs that use a hashmap to manage the pool
of unique, interned strings.

Note: strintern(getenv()) could be used to sanitize git's use of getenv(),
in case we ever encounter a platform where a call to getenv() invalidates
previous getenv() results (which is allowed by POSIX).

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hashmap.h

@@ -87,4 +87,12 @@ static inline void *hashmap_iter_first(struct hashmap *map,
 	return hashmap_iter_next(iter);
 }
 
+/* string interning */
+
+extern const void *memintern(const void *data, size_t len);
+static inline const char *strintern(const char *string)
+{
+	return memintern(string, strlen(string));
+}
+
 #endif