raco pkg clone: avoid fetching by commit id

A commit id apparently doesn't work in place of a <refexp> in some
versions of Git.
repair a test

The test tried to detect separate evaluations by using the
result of `(current-inexact-milliseconds)`, but a clock's
resolution might not be high enough. Use `(gensym)`, instead.
raco pkg update: better handling of a missing dependency

When updating a linked package whose dependencies have changed,
and when a dependency is missing, then effectively reinstall
the link to get updates as requested by the user.
raco pkg: default `--deps` to `search-ask` in interactive mode

The implemented default for `raco pkg update` actually depended on the
way that a package is installed, and it's difficult to reason about or
to implement the default that is suggested by the documentation.
Meanwhile, `search-ask` seems the most sensible always in interactive
mode (now that we have a way to specify batch mode).