backport: isnegative in intpow

[codecov[bot]]

Codecov Report

Patch coverage: 75.00% and project coverage change: -0.03% :warning:

Comparison is base (e25ab77) 71.30% compared to head (09341a9) 71.27%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## backport_0.8 #591 +/- ## ================================================ - Coverage 71.30% 71.27% -0.03% ================================================ Files 81 81 Lines 8591 8593 +2 ================================================ - Hits 6126 6125 -1 - Misses 2465 2468 +3 ``` | [Files Changed](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/591?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/Fun.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/591?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL0Z1bi5qbA==) | `67.67% <75.00%> (-0.12%)` | :arrow_down: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/591/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation)

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