ignore: fix filtering searching subdir or .ignore in parent dir

[WalterScottYoung]

The previous code deleted too many parts of the path when constructing the absolute path, resulting in a shortened final path. This patch creates the correct absolute path by only removing the necessary parts.

Fixes #2836 and #2747 and #2731

[WalterScottYoung]

One thing to notice is that commit cad1f5f didn't seems to fully fix #1757, the test in issue 1757 will fail if we add one more directory in the path. I don't know if we should reopen 1757 or something like that to note others about this?

The following test from 1757 with "onemore" directory will fail before this fix.

mkdir tmp
cd tmp
mkdir rust
mkdir -p rust/target/onemore
echo needle > rust/source.rs
echo needle > rust/target/onemore/rustdoc-output.html
echo rust/target/onemore > .ignore
rg --files-with-matches needle
rg --files-with-matches needle rust
echo target >> .ignore
rg --files-with-matches needle rust