Re: git status: small difference between stating whole repository and small subdirectory

Jeff King [off-list ref] writes:

Interestingly, on my git.git repo, I had an empty cache. Running "git
read-tree HEAD" filled it (according to test-dump-cache-tree). It seems
that running "git checkout" empties the cache.  So perhaps git could do
better about keeping the cache valid over time.

At least in the early days unpack-trees built the result by manually
adding an entry without calling the add_index_entry() all over the place,
which meant that it was futile to pretend that there is even a slight
chance that complex beast would correctly invalidate cached tree
information at all the necessary places. I recall that I added a code to
nuke the cache tree at the very beginning of "merging" codepaths to avoid
any bogus cache tree result to be stored in the resulting index.

These days, we have src_index and dst_index, and dst_index IIRC can start
as empty in which case "start from kept information and selectively
invalidate" would not work at all.  When src_index and dst_index are the
same, however, you should be able to keep the cached tree valid, at least
in theory.