From: Henrique Ferreiro <hferreiro@igalia.com>

Diffing the working tree against a commit with a pathspec can take
time quadratic in the size of the index when the pathspec matches a
subtree whose entries are the first entries of the index.  Fix it by
having next_cache_entry() record how far it scanned in cache_bottom,
so repeated calls no longer rescan the growing prefix of
already-unpacked entries.  On a Chromium checkout (~500k index
entries),

	git diff HEAD -- .agents/OWNERS

took about 8 minutes before this change and 0.07 seconds after it.
The same diff without the commit, without the pathspec, or with
--cached was already instant.

Add p0009-diff-pathspec.sh, which builds a 100,000-entry index whose
first path lives in a subtree, to guard against the regression.
Comparing v2.55.0 with this change:

Test                            v2.55.0           HEAD
---------------------------------------------------------------------

Submitted-As: https://lore.kernel.org/git/pull.2353.git.git.1783458106037.gitgitgadget@gmail.com