Include path option is relative to the main folder.

[BeniaminK]

Due to this commit we might be sure that the folder structure is considered different when files are in the different subdirectories. For example:

Environment:

> mkdir -p /tmp/subdir
> touch /tmp/subdir/a.txt

Was:

> checksumdir -p /tmp
74be16979710d4c4e7c6647856088456
> checksumdir -p /tmp/subdir
74be16979710d4c4e7c6647856088456

The hashes are the same although we are checking /tmp/subdir/a.txt in first case and subdir/a.txt in the latter.

Now it will be:

> checksumdir -p /tmp
f956ecf8ffe9798427114d9b69400ce5
> checksumdir -p /tmp/subdir
0ec539fca957e328003429f8f741c7b1

Different checksum, because the relative paths are different.

This way we may find the same folder structure.