khash: implement begin/end via functions instead of macros

Right now, the `git_*map_begin()` and `git_*map_end()` helpers are
implemented via macros which simply redirect to `kh_begin` and `kh_end`.
As these macros refer to members of the map structures, they make it
impossible to move the khash include into the implementation files.

Implement these helpers as real functions instead to further decouple
the headers from implementations.

src/offmap.c

@@ -81,3 +81,14 @@ void git_offmap_delete(git_offmap *map, const git_off_t key)
 	if (git_offmap_valid_index(map, idx))
 		git_offmap_delete_at(map, idx);
 }
+
+size_t git_offmap_begin(git_offmap *map)
+{
+	GIT_UNUSED(map);
+	return 0;
+}
+
+size_t git_offmap_end(git_offmap *map)
+{
+	return map->n_buckets;
+}

src/offmap.h

@@ -40,6 +40,9 @@ int git_offmap_put(git_offmap *map, const git_off_t key, int *err);
 void git_offmap_insert(git_offmap *map, const git_off_t key, void *value, int *rval);
 void git_offmap_delete(git_offmap *map, const git_off_t key);
 
+size_t git_offmap_begin(git_offmap *map);
+size_t git_offmap_end(git_offmap *map);
+
 #define git_offmap_foreach		kh_foreach
 #define git_offmap_foreach_value	kh_foreach_value
 

src/oidmap.c

@@ -103,3 +103,14 @@ void git_oidmap_delete(git_oidmap *map, const git_oid *key)
 	if (git_oidmap_valid_index(map, idx))
 		git_oidmap_delete_at(map, idx);
 }
+
+size_t git_oidmap_begin(git_oidmap *map)
+{
+	GIT_UNUSED(map);
+	return 0;
+}
+
+size_t git_oidmap_end(git_oidmap *map)
+{
+	return map->n_buckets;
+}

src/oidmap.h

@@ -43,9 +43,9 @@ int git_oidmap_put(git_oidmap *map, const git_oid *key, int *err);
 void git_oidmap_insert(git_oidmap *map, const git_oid *key, void *value, int *rval);
 void git_oidmap_delete(git_oidmap *map, const git_oid *key);
 
-#define git_oidmap_foreach_value kh_foreach_value
+size_t git_oidmap_begin(git_oidmap *map);
+size_t git_oidmap_end(git_oidmap *map);
 
-#define git_oidmap_begin	kh_begin
-#define git_oidmap_end		kh_end
+#define git_oidmap_foreach_value kh_foreach_value
 
 #endif

src/strmap.c

@@ -102,6 +102,17 @@ void git_strmap_delete(git_strmap *map, const char *key)
 		git_strmap_delete_at(map, idx);
 }
 
+size_t git_strmap_begin(git_strmap *map)
+{
+	GIT_UNUSED(map);
+	return 0;
+}
+
+size_t git_strmap_end(git_strmap *map)
+{
+	return map->n_buckets;
+}
+
 int git_strmap_next(
 	void **data,
 	git_strmap_iter* iter,

src/strmap.h

@@ -45,8 +45,8 @@ void git_strmap_delete(git_strmap *map, const char *key);
 #define git_strmap_foreach		kh_foreach
 #define git_strmap_foreach_value	kh_foreach_value
 
-#define git_strmap_begin		kh_begin
-#define git_strmap_end		kh_end
+size_t git_strmap_begin(git_strmap *map);
+size_t git_strmap_end(git_strmap *map);
 
 int git_strmap_next(
 	void **data,