files-backend: convert git_path() to strbuf_git_path()

files-backend: convert git_path() to strbuf_git_path()

git_path() and friends are going to be killed in files-backend.c in near
future. And because there's a risk with overwriting buffer in
git_path(), let's convert them all to strbuf_git_path(). We'll have
easier time killing/converting strbuf_git_path() then because we won't
have to worry about memory management again.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>