refspec: set err message on invalid refspec

If a refspec could not be parsed, the git_refspec__parse function would
return an error value but would not provide additional error information
for the callers. This commit amends that function to set a more useful
error message.

src/refspec.c

@@ -133,6 +133,9 @@ int git_refspec__parse(git_refspec *refspec, const char *input, bool is_fetch)
 	return 0;
 
  invalid:
+        giterr_set(
+                GITERR_INVALID,
+                "'%s' is not a valid refspec.", input);
 	return -1;
 }