Fix speed regression

...since 8d443f62c, faster than 1.3.4 now, fixes issue #280 

Thanks to @ladislas for spotting the problem and pointing towards the culprit of variable assignment types.

Audited *some* use of := vs. =, when calling shell/foreach/wildcard, could probably do with more looking into.

```
1.3.4
real	0m2.963s
real	0m2.948s

master
real	0m3.060s
real	0m3.047s

PR
real	0m2.792s
real	0m2.799s
real	0m2.789s
real	0m2.790s
real	0m2.793s
```