Ignore files and folders listed in .gitignore and meta/inspect-ignore

The inspect tool gives a lot of false positives when I run it locally on my computer. These would disappear if it would ignore what is in .gitignore.

Once this feature is added, we could generally switch to a solution where the inspect tool scans the meta folder and reads a `inspect-ignore` file. This would have the same format as .gitignore, so the same code in the inspect tool can be used to read both. This would be a better solution than using boost-no-inspect files and "boost-no-inspect" comments.