tagpreview.sh should have an option NOT to modify file paths

- [x] I have fzf 0.23.0 or above
- [x] I have read through https://github.com/junegunn/fzf.vim/blob/master/README.md
- [x] I have read through https://github.com/junegunn/fzf/blob/master/README-VIM.md
- [x] I have read through the manual page of fzf (`man fzf`)
- [x] I have searched through the existing issues

I'm trying to add a preview feature to fzf-tags: https://github.com/zackhsi/fzf-tags/issues/16

fzf-tags uses vim's taglist(), and when there are multiple matches it uses fzf to select between the matches. It assigns an index to each match, identical to how g] (:tselect) would number them. Then it uses :tag to jump to the selected tag.

taglist() returns the appropriate file path to read the file in which the tag shows up. Depending on the location of the tagfile, it may return a relative, or an absolute path (I noticed it returning an absolute path when the tagfile is above the current working directory in the filesystem hierarchy). In any case, the filename 'fixing' in tagpreview.sh which is harmful.