cffi callbacks written shorter

If the callback is defined with a typedef in decl, don't need to write
again the signature in Python, just the name.

pygit2/__init__.py

@@ -153,7 +153,7 @@ def init_repository(path, bare=False,
     # Ok
     return Repository(to_str(path))
 
-@ffi.callback('int (*cb)(git_repository **out, const char *path, int bare, void *payload)')
+@ffi.callback('git_repository_create_cb')
 def _repository_create_cb(repo_out, path, bare, data):
     d = ffi.from_handle(data)
     try:
@@ -167,8 +167,7 @@ def _repository_create_cb(repo_out, path, bare, data):
 
     return 0
 
-@ffi.callback('int (*cb)(git_remote **out, git_repository *repo,'
-              'const char *name, const char *url, void *payload)')
+@ffi.callback('git_remote_create_cb')
 def _remote_create_cb(remote_out, repo, name, url, data):
     d = ffi.from_handle(data)
     try:

pygit2/remote.py

@@ -252,8 +252,9 @@ def _sideband_progress_cb(string, length, data):
 
         return 0
 
-    @ffi.callback('int (*update_tips)(const char *refname, const git_oid *a,'
-                  'const git_oid *b, void *data)')
+    @ffi.callback(
+        'int (*update_tips)(const char *refname, const git_oid *a, const git_oid *b, void *data)'
+    )
     def _update_tips_cb(refname, a, b, data):
         self = ffi.from_handle(data)
 
@@ -273,7 +274,7 @@ def _update_tips_cb(refname, a, b, data):
 
         return 0
 
-    @ffi.callback("int (*push_update_reference)(const char *ref, const char *msg, void *data)")
+    @ffi.callback('git_push_update_reference_cb')
     def _push_update_reference_cb(ref, msg, data):
         self = ffi.from_handle(data)
 
@@ -291,9 +292,7 @@ def _push_update_reference_cb(ref, msg, data):
 
         return 0
 
-    @ffi.callback('int (*cb)(git_cred **cred, const char *url,'
-                  'const char *username_from_url, unsigned int allowed_types,'
-                  'void *data)')
+    @ffi.callback('git_cred_acquire_cb')
     def _credentials_cb(cred_out, url, username, allowed, data):
         self = ffi.from_handle(data)
 
@@ -312,7 +311,7 @@ def _credentials_cb(cred_out, url, username, allowed, data):
 
         return 0
 
-    @ffi.callback('int (*cb)(git_cert *cert, int valid, const char *host, void *payload)')
+    @ffi.callback('git_transport_certificate_check_cb')
     def _certificate_cb(cert_i, valid, host, data):
         self = ffi.from_handle(data)