Print the error message reported by the remote server when failing to update a branch (for e.g. because of a server-side check.) Reported by gonzalo@, with help and ok stsp@.

ok stsp@

ok op@

ok op@

Convert from st.m_time (second-precision time_t) to st.m_tim (struct timespec). To compensate for the potential case where a filesystem provides resolution in seconds only, always read the directory if no pack files are known to exist. Otherwise, there is a race condition when gotd repo_write creates a new pack and a request arrives for repo_read shortly after. Caught by a regression test for gotd on empty repositories. Test failure pointed out by Mikhail. ok op@

ok op@