Add filetype to popups

[gabs712]

Description

gitsigns.preview_hunk and gitsigns.blame_line popups currently doesn't have filetypes.
I suggest adding one for each, so them can be referenced.

Neovim version

v0.11.0

Operating system and version

Ubuntu 24.04.1

Expected behavior

No response

Actual behavior

Running :set ft? inside of popups outputs a null value.

Minimal config

require('gitsigns').setup()

Steps to reproduce

1. Trigger gitsigns.blame_line or gitsigns.preview_hunk
2. Trigger it again to enter the window
3. Run :set ft?
4. There isn't a filetype defined

Gitsigns debug messages

No response

Gitsigns cache

No response