Fix NameError when encountering errors during HTTP operation. (Jelmer Vernooij, #1208)

Raise exception when default identity can't be found. (Jelmer Vernooij)

Add a dedicated exception class for unresolved deltas. (Jelmer Vernooij, #1221)

Support credentials in proxy URL. (Jelmer Vernooij, #1227)

Add dulwich.porcelain.for_each_ref. (Daniele Trifirò)