Rework the schell script

* optimize the performance (see above)
* remove the seq usage (no used, an display some errors, see #11)
* use `echo` in a simpler way

some bench:

Before my work:

```
>(17)[~/dev/tools/ArcheoloGit](speedup %) time ./run.sh ../../labs/twig 2>/dev/null
Number of files: 554
>100%

real    0m43.977s
user    0m35.364s
sys     0m4.624s
```

With only one git call by iteration

```
>(17)[~/dev/tools/ArcheoloGit](speedup *%) time ./run.sh ../../labs/twig
Number of files: 554
>100%

real    0m31.167s
user    0m25.316s
sys     0m2.808s
```

With only one git call by iteration + the replacement of the while by a for

```
>(17)[~/dev/tools/ArcheoloGit](speedup *%) time ./run.sh ../../labs/twig
Number of files: 554
>100%

real    0m30.606s
user    0m24.820s
sys     0m2.628s
```
Merge pull request #12 from lyrixx/speedup

Rework the schell script