tree::walk: Allow callback to return an int

`git_tree_walk` in libgit2 accepts a callback which can return an int.
The int tells the walker whether to recurse on a tree object or not.

This is useful behaviour if you don't want to walk an *entire* large
tree.

To upgrade existing code, just change existing callbacks to return `int`
instead of `void`, and add a `return 0;`

Author: craigds

README.md

@@ -1,3 +1,11 @@
+# cppgit2: Koordinates Fork
+
+This fork is maintained by Koordinates for the purposes of implementing [Kart](https://kartproject.org).
+
+This fork is likely to be (somewhat) actively maintained, so someone will respond to issues/PRs.
+
+# cppgit2
+
 <p align="center">
   <img height="100" src="img/logo.png"/>
 </p>
@@ -314,6 +322,7 @@ int main(int argc, char **argv) {
                   }
                   std::cout << type_string << " [" << entry.id().to_hex_string(8)
                             << "] " << entry.filename() << std::endl;
+                  return 0;
               });
 
   } else {

include/cppgit2/tree.hpp

@@ -140,8 +140,10 @@ class tree : public libgit2_api {
   class repository owner() const;
 
   // Traverse the entries in a tree and its subtrees in post or pre order.
+  // If the callback returns a positive value, the passed entry will be skipped
+  // on the traversal (in pre mode). A negative value stops the walk.
   void walk(traversal_mode mode,
-            std::function<void(const std::string &, const tree::entry &)>
+            std::function<int(const std::string &, const tree::entry &)>
                 visitor) const;
 
   enum class update_type {

samples/walk_tree.cpp

@@ -31,6 +31,7 @@ int main(int argc, char **argv) {
                 }
                 std::cout << type_string << " [" << entry.id().to_hex_string(8)
                           << "] " << entry.filename() << std::endl;
+                return 0;
               });
 
   } else {

src/tree.cpp

@@ -64,10 +64,10 @@ size_t tree::size() const { return git_tree_entrycount(c_ptr_); }
 repository tree::owner() const { return repository(git_tree_owner(c_ptr_)); }
 
 void tree::walk(traversal_mode mode,
-                std::function<void(const std::string &, const tree::entry &)>
+                std::function<int(const std::string &, const tree::entry &)>
                     visitor) const {
   struct visitor_wrapper {
-    std::function<void(const std::string &, const tree::entry &)> fn;
+    std::function<int(const std::string &, const tree::entry &)> fn;
   };
 
   visitor_wrapper wrapper;
@@ -76,8 +76,7 @@ void tree::walk(traversal_mode mode,
   auto callback_c = [](const char *root, const git_tree_entry *entry,
                        void *payload) {
     auto wrapper = reinterpret_cast<visitor_wrapper *>(payload);
-    wrapper->fn(root ? std::string(root) : "", tree::entry(entry));
-    return 0;
+    return wrapper->fn(root ? std::string(root) : "", tree::entry(entry));
   };
 
   if (git_tree_walk(c_ptr_, static_cast<git_treewalk_mode>(mode), callback_c,