fix: scan `.git/config` for secrets

[knqyf263]

Description

Trivy currently skips **/.git for efficiency.

trivy/pkg/fanal/walker/walk.go

Line 18 in 88702cf

"**/.git",

However, .git/config could sometimes include credentials (see #5180 (comment)). These directories shouldn't be skipped.

[sneaky-potato]

Hi can I work on this issue?