Update iterators for consistency across library

This updates all the `foreach()` type functions across the library
that take callbacks from the user to have a consistent behavior.
The rules are:

* A callback terminates the loop by returning any non-zero value
* Once the callback returns non-zero, it will not be called again
  (i.e. the loop stops all iteration regardless of state)
* If the callback returns non-zero, the parent fn returns GIT_EUSER
* Although the parent returns GIT_EUSER, no error will be set in
  the library and `giterr_last()` will return NULL if called.

This commit makes those changes across the library and adds tests
for most of the iteration APIs to make sure that they follow the
above rules.

1 files changed, 2 insertions, 2 deletions

diff --git a/include/git2/config.h b/include/git2/config.h
index c46e7fc9d..8a36885c7 100644
--- a/include/git2/config.h
+++ b/include/git2/config.h
@@ -302,12 +302,12 @@ GIT_EXTERN(int) git_config_delete(git_config *cfg, const char *name);
  * The callback receives the normalized name and value of each variable
  * in the config backend, and the data pointer passed to this function.
  * As soon as one of the callback functions returns something other than 0,
- * this function returns that value.
+ * this function stops iterating and returns `GIT_EUSER`.
  *
  * @param cfg where to get the variables from
  * @param callback the function to call on each variable
  * @param payload the data to pass to the callback
- * @return 0 or the return value of the callback which didn't return 0
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_config_foreach(
 	git_config *cfg,