sparse-checkout: use hashmaps for cone patterns

sparse-checkout: use hashmaps for cone patterns

The parent and recursive patterns allowed by the "cone mode"
option in sparse-checkout are restrictive enough that we
can avoid using the regex parsing. Everything is based on
prefix matches, so we can use hashsets to store the prefixes
from the sparse-checkout file. When checking a path, we can
strip path entries from the path and check the hashset for
an exact match.

As a test, I created a cone-mode sparse-checkout file for the
Linux repository that actually includes every file. This was
constructed by taking every folder in the Linux repo and creating
the pattern pairs here:

	/$folder/
	!/$folder/*/

This resulted in a sparse-checkout file sith 8,296 patterns.
Running 'git read-tree -mu HEAD' on this file had the following
performance:

    core.sparseCheckout=false: 0.21 s (0.00 s)
     core.sparseCheckout=true: 3.75 s (3.50 s)
 core.sparseCheckoutCone=true: 0.23 s (0.01 s)

The times in parentheses above correspond to the time spent
in the first clear_ce_flags() call, according to the trace2
performance traces.

While this example is contrived, it demonstrates how these
patterns can slow the sparse-checkout feature.

Helped-by: Eric Wong <e@80x24.org>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>