If a merge is in progress, invoke 'callback' for each commit ID in the
MERGE_HEAD file.
Return a non-zero value from the callback to stop the loop.
@param repo A repository object
@param callback Callback function
@param payload Pointer to callback data (optional)
@return 0 on success, non-zero callback return value, git_error_code.GIT_ENOTFOUND if
there is no MERGE_HEAD file, or other error code.
If a merge is in progress, invoke 'callback' for each commit ID in the MERGE_HEAD file.
Return a non-zero value from the callback to stop the loop.
@param repo A repository object @param callback Callback function @param payload Pointer to callback data (optional) @return 0 on success, non-zero callback return value, git_error_code.GIT_ENOTFOUND if there is no MERGE_HEAD file, or other error code.