builtin/sparse-checkout: add check-rules command

There exists no direct way to interrogate git about which paths are
matched by a given set of sparsity rules. It is possible to get this
information from git, but it includes checking out the commit that
contains the paths, applying the sparse checkout patterns and then using
something like 'git ls-files -t' to check if the skip worktree bit is
set. This works in some case, but there are cases where it is awkward or
infeasible to generate a checkout for this purpose.

Exposing the pattern matching of sparse checkout enables more tooling to
be built and avoids a situation where tools that want to reason about
sparse checkouts start containing parallel implementation of the rules.
To accommodate this, add a 'check-rules' subcommand to the
'sparse-checkout' builtin along the lines of the 'git check-ignore' and
'git check-attr' commands. The new command accepts a list of paths on
stdin and outputs just the ones the match the sparse checkout.

To allow for use in a bare repository and to allow for interrogating
about other patterns than the current ones, include a '--rules-file'
option which allows the caller to explicitly pass sparse checkout rules
in the format accepted by 'sparse-checkout set --stdin'.

To allow for reuse of the handling of input patterns for the
'--rules-file' flag, modify 'add_patterns_from_input()' to be able to
read from a 'FILE' instead of just stdin.

To allow for reuse of the logic which decides whether or not rules
should be interpreted as cone-mode patterns, split that part out of
'update_modes()' such that can be called without modifying the config.

An alternative could have been to create a new 'check-sparsity' command.
However, placing it under 'sparse-checkout' allows for a) more easily
re-using the sparse checkout pattern matching and cone/non-code mode
handling, and b) keeps the documentation for the command next to the
experimental warning and the cone-mode discussion.

Signed-off-by: William Sprent <williams@unity3d.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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