and adjust the callers. discussed with and ok stsp@

We fail to release the memory for the nodes. To fix it however, we some consumer of the commit graph iterator need to be corrected: the returned pointer is safe to be used only up until the next iter_next call; save a copy it if it's needed afterwards too. ok stsp@

Avoids leaking the node structure returned from add_node(). ok op@

while here drop the ternary too: if add_node succeeds `new_node' is non-NULL. ok stsp@

ok stsp@

The tailq should ideally be empty when we reach got_commit_graph_close, but consumer code may exit the commit graph loop earlier. spotted by and ok stsp@

Saves us from doing a malloc/free call for every item on the list. ok op@

Instead of internally opening and closing the same commit object over and over again, require callers to pass an open commit object in. Avoids an inherent dependency on the commit object cache for reasonable performance. ok op@

The singly-linked tail queue macros were added to OpenBSD 6.9 and are more widely available on other systems. ok stsp