Problem:
No lint for strtol().
Solution:
Add lint, and update existing usages.
Callers that previously got a (garbage) large positive value:
- `indent.c`: use getdigits() (intmax, no int truncation) with def=1 so
overflow/too-large values stay positive and fall through to the
existing "too big" check (E475).
- `file_search.c`: use getdigits() with def=255 so overflow/too-large
values keep the "max expand" (else) branch.
The other conversions (api/command.c, eval/window.c, highlight_group.c,
tui/tui.c) only diverge on pathological overflow inputs where strtol's
result was already garbage and the observable outcome is unchanged.
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