Add missing srcs and update got_repo_read_gitconfig() to be consistent with recent changes. ok stsp@

Only fetch the work tree's branch if the -b option is not specified. This keeps -b functional as an override when invoked in a work tree. Our previous changes did not consider that got.conf is also a source of lists of branches to fetch, and that -b is supposed to work as an override of any default behaviour. We were implicitly appending the work tree's branch as if it was mentioned as an override on the command line, which was wrong and based on a misunderstanding of the intended behaviour. Without -b on the command line we obtain a list of branches to fetch from got.conf and use this list if it is not empty. The repository's HEAD will be fetched only if neither the -b option, nor got.conf, nor a work tree tell us what to fetch. Make the man page more clear by moving the explanation of the default behaviour into the main section of 'got fetch', leaving the -a and -b option descriptions free of such details. ok jamsek

it describes bump_base_commit_id_everywhere, not its per-entry callback bump_base_commit_id.

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@