Re: [PATCH v6 1/1] builtin/grep.c: integrate with sparse index

Shaoxuan Yuan [off-list ref] writes:

- Command used for testing:

	git grep --cached bogus -- "f2/f1/f1/*"

The reason for specifying a pathspec is that, if we don't specify a
pathspec, then grep_tree() will walk all the trees and blobs to find the
pattern, and the time consumed doing so is not too different from using
the original ensure_full_index() method, which also spends most of the
time walking trees. However, when a pathspec is specified, this latest
logic will only walk the area of trees enclosed by the pathspec, and the
time consumed is reasonably a lot less.

Good.  So without pathspec, we lazily populate the index and catch
matches even from outside the sparse cone.  We punt to "implicitly"
apply the sparse cone(s) as a pathspec that limits the hits to the
paths in the sparse cone(s).

That is, if we don't specify a pathspec, the performance difference [1]
is indistinguishable: both methods walk all the trees and take generally
same amount of time (even with the index construction time included for
ensure_full_index()).

Good.