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_repo_get_gitconfig_extensions is only used in gotadmin to check if the preciousObjects extension is active; let's replace it with a function that just checks whether a certain extension is active. It simplifies future changes to the extensions handling. ok stsp@

after got_object_id_hex was introduced and used in error.c, the building of delta, deltify and path failed due to missing symbols. Spotted by stsp and tracey, ok stsp

ok stsp@

ok stsp@

It's an analogous to got_object_id_str but writes to the given buffer. ok + improvements by stsp@

ok stsp@

While here, for consistency, check dup() return value for -1 rather than < 0. ok stsp@

ok stsp@

serializes an id to a local buffer before before returning. ok stsp

writing everything down to avoid forgetting stuff...