treewide: rename 'struct exclude_list' to 'struct pattern_list'

treewide: rename 'struct exclude_list' to 'struct pattern_list'

The first consumer of pattern-matching filenames was the
.gitignore feature. In that context, storing a list of patterns
as a 'struct exclude_list'  makes sense. However, the
sparse-checkout feature then adopted these structures and methods,
but with the opposite meaning: these patterns match the files
that should be included!

It would be clearer to rename this entire library as a "pattern
matching" library, and the callers apply exclusion/inclusion
logic accordingly based on their needs.

This commit renames 'struct exclude_list' to 'struct pattern_list'
and renames several variables called 'el' to 'pl'.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>