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Re: [GSoC][RFC] discussion about stashing with conflicts

while understanding add_index_entry_with_check()
i got to do_invalidate_path().

the commit hash for do_invalidate_path() function is

in the commit message it is explained that, cache-tree is used to
store object names of index file objects and it is kept separate from
index file because adding it would change index file format.

the part i couldn't understand is:
"During various index manupulations, we selectively invalidate
the parts so that the next write-tree can bypass regenerating
tree objects for unchanged parts of the directory hierarchy."

what exactly does invalidating means here ?