strmap: add more utility functions

This adds a number of additional convienence functions I want/need:
  * strmap_get_size()
  * strmap_empty()
  * strmap_remove()
  * strmap_for_each_entry()
  * strmap_get_entry()

I suspect the first four are self-explanatory.

strmap_get_entry() is similar to strmap_get() except that instead of just
returning the void* value that the string maps to, it returns the
strmap_entry that contains both the string and the void* value (or
NULL if the string isn't in the map).  This is helpful because it avoids
multiple lookups, e.g. in some cases a caller would need to call:
  * strmap_contains() to check that the map has an entry for the string
  * strmap_get() to get the void* value
  * <do some work to update the value>
  * strmap_put() to update/overwrite the value
If the void* pointer returned really is a pointer, then the last step is
unnecessary, but if the void* pointer is just cast to an integer then
strmap_put() will be needed.  In contrast, one can call strmap_get_entry()
and then:
  * check if the string was in the map by whether the pointer is NULL
  * access the value via entry->value
  * directly update entry->value
meaning that we can replace two or three hash table lookups with one.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strmap.c

@@ -87,6 +87,11 @@ void *strmap_put(struct strmap *map, const char *str, void *data)
 	return old;
 }
 
+struct strmap_entry *strmap_get_entry(struct strmap *map, const char *str)
+{
+	return find_strmap_entry(map, str);
+}
+
 void *strmap_get(struct strmap *map, const char *str)
 {
 	struct strmap_entry *entry = find_strmap_entry(map, str);
@@ -97,3 +102,18 @@ int strmap_contains(struct strmap *map, const char *str)
 {
 	return find_strmap_entry(map, str) != NULL;
 }
+
+void strmap_remove(struct strmap *map, const char *str, int free_value)
+{
+	struct strmap_entry entry, *ret;
+	hashmap_entry_init(&entry.ent, strhash(str));
+	entry.key = str;
+	ret = hashmap_remove_entry(&map->map, &entry, ent, NULL);
+	if (!ret)
+		return;
+	if (free_value)
+		free(ret->value);
+	if (map->strdup_strings)
+		free((char*)ret->key);
+	free(ret);
+}