bump hash size and add algo field to got_object_id; fix a few places

add field to got_packidx to propagate the hash algo allows to drop all the `algo' hardcodings in lib/pack.c and pack_idx.c

add an hash algo field to got_pack - maybe useless

fix interaction of 'got fetch -b', got.conf, and work tree Without branches in got.conf for a remote, and without -b/-R options, the fallback to HEAD would only work when not invoked in a work tree. With this fix 'got fetch' should behave as described in the man page. The -b option now overrides both got.conf and the fallback to the work tree's branch. And fallback to HEAD works as expected when invoked in a repository. Also, do not strictly require remote repositories to provide a branch from the refs/heads/ namespace. In such cases users should be able to use -R to select something to fetch. ok jamsek

got-read-gitconfig: send key-value pairs for extensions Most extension allow only for a boolean value so the current behaviour of just sending the extension with a trueish value is fine. However, some extensions that we could eventually support (like "objectformat") have a string value. This is a preparatory step towards that. ok stsp@

got_imsg_commit_object: use struct instead of buffer for id ok stsp@

got_imsg_object: use struct instead of buffer for id ok stsp@

got_imsg_send_remote_ref: use struct instead of buffer for id ok stsp@

got_imsg_send_ref: use struct instead of buffer for id ok stsp@

got_imsg_fetch_ref: use struct instead of buffer for id ok stsp@

got_imsg_fetch_have_ref: use struct instead of buffer for id ok stsp@

got_imsg_raw_delta: use struct instead of buffer for id ok stsp@

got_imsg_raw_delta_request: use struct instead of buffer for id ok stsp@

got_imsg_packed_object: use struct instead of buffer for id ok stsp@

avoid copying reused deltas to delta cache file; copy from pack file instead ok op@