Implement stsp's suggestion to only fetch remote's HEAD if the symref refs/remote/*/HEAD exists, and its target no longer matches the remote HEAD. This ensures users tracking a project won't miss a change in HEAD, while also fixing the issue reported by naddy where HEAD was fetched by default even though a specific, potentially less active, branch is cloned, resulting in a repository with more commits than necessary. In addition, unless 'got fetch -b <branch>' is used, the remote HEAD branch will be fetched if branches are not set in got.conf and there is no work tree to ascertain a branch, or said branches are not found on the server. ok stsp@

As discussed on irc, this drops the implicit remote HEAD fetch when -b is used. got.1 has been updated to make the new fetch behaviour clear. ok stsp@

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

Both GOT_FETCH_PKTMAX and GOT_SEND_PKTMAX had the same value. Declare this value as GOT_PKT_MAX in got_lib_pkt.h instead.

These functions are used by 'got send' and 'got fetch' in order to open network connections to a server. Move them into new file lib/dial.c and declare relevant functions in got_dial.h and lib/got_lib_dial.h. No functional change.