treewide: rename 'struct exclude' to 'struct path_pattern'

treewide: rename 'struct exclude' to 'struct path_pattern'

The first consumer of pattern-matching filenames was the
.gitignore feature. In that context, storing a list of patterns
as a list of 'struct exclude' items 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' to 'struct path_pattern'
and renames several variable names to match. 'struct pattern'
was already taken by attr.c, and this more completely describes
that the patterns are specific to file paths.

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