Don't use '==' for equality matching in sh, as this won't work across all shells. ok @naddy

Since the interpreter for the regress shell scripts are using /bin/sh this will usually imply some level of POSIX compliance (that isn't bash-specific, for instance). Some systems use dash as their sh shell and as such is stricter POSIX compliance. To help -portable, make the shell test checks use a single '=' for equality, rather than '=='.

The letter "o" does not overlap with existing options of 'got ref' and is also used by mutt(1) for this purpose.

This fixes an issue where some references were missing from lists of references sorted by timestamp.

Move a small amount of code from worktree.c to a new file worktree_open.c, which contains everything required to open and close a work tree and inspect some of its basic parameters. This can be used by gotadmin. ok tracey