tool: cfg.GroupSubdirBy - group by dir or fname

[xushiwei]

No description provided.

[fennoai]

Review: add GroupSubdirBy config option

The change is small, focused, and clean. Replacing the goFileOf bool parameter with a named iota enum (groupSubdirByNone/Dir/Fname) reads clearly, the empty-string + GroupSubdir=true backward-compat path is handled explicitly, and the default warning on bad input is a sensible defensive choice. No performance or clearly-exploitable security concerns were found.

A few points below are worth confirming or tightening before merge — mostly documentation clarity and one backward-compat edge case.

Minor notes (not inlined):

• No unit tests exercise goFileOf's three branches or the config-to-enum switch. Given the "fname" collision semantics and the pos > 0 vs pos >= 0 asymmetry, a small table-driven test (top-level file, nested file, shared basename, leading-_ file, extensionless file × each enum value) would cheaply lock in intended behavior.
• The accepted values "dir"/"fname" live only as inline string literals; defining them as named constants shared by the switch, the warning message, and the doc comment would avoid future drift.