Teach git-diff-tree(1) option --max-depth

Please consider these patches authored by Peff. They add option --max-depth to the diff machinery.

During the process to upstream the git-blame-tree(1), later named git-last-modified(1), various times1 the topic was raised to add option --max-depth to the diff machinery. In this patch series this change is added as a separate patch series.

The patches were originally written by Peff3 and I'm crediting him as the author. I've taken the patches almost verbatim from his fork on GitHub, with some minor tweaks in the commit messages. Because only tiny changes were made, I've kept his Signed-off-by trailers, but I can remove if you disagree.

The goal of the option --max-depth is to stop recursively traversing the tree if the given depth is reached from the pathspec.

These patches add max_depth and max_depth_valid to struct diff_options. This is different from what git-grep(1) does, which uses max_depth on struct pathspec instead. At the moment I'm on the fence whether this is an issue: while it probably makes sense to consolidate them into the same structs, it does not really make sense to reuse these the struct fields if they are used in two separate code paths.

Cc: Jeff King peff@peff.net

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{ "series": { "revision": 1, "change-id": "20250724-toon-max-depth-25d3c19e2607", "prefixes": [] } }