How does Git detect file changes without scanning every file on disk?
Git builds a Merkle tree: files are hashed into blob objects, directories into tree objects, and trees into a root tree. If a tree's hash matches the previous commit, everything inside is provably unchanged. Git skips entire subtrees, descending only where hashes differ. This reduces checking millions of files to comparing a handful of hashes.
Answered in
Merkle Trees: How Git Detects Changes in MillisecondsGit hashes files into nested cryptographic trees to skip unchanged directories in one comparison, finding changes across millions of files faster than scanning.
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