bug#59087: 29.0.50; package-vc-update error

[Feng Shu]

package-vc-prepare-patch seem to run in wrong directory:

```
Debugger entered--Lisp error: (error "Buffer *Packages* is not associated with 
a file")
  signal(error ("Buffer *Packages* is not associated with a file"))
  error("Buffer %s is not associated with a file" "*Packages*")
  vc-ensure-vc-buffer()
  vc-prepare-patch("Feng Shu <tumashu@163.com>" nil ("HEAD"))
  package-vc-prepare-patch(#s(package-desc :name pyim :version (5 2 8) :summary 
"A Chinese input method support quanpin, shuangpin,..." :reqs ((emacs (25 1)) 
(async (1 6)) (xr (1 13))) :kind vc :archive nil :dir 
"/home/feng/.emacs.d/elpa-29/pyim" :extras ((:commit . 
"93fab3c9e8c2490687781d45c1cecead0bd3ecf2") (:authors ("Ye Wenbin" . 
"wenbinye@163.com") ("Feng Shu" . "tumashu@163.com")) (:maintainer "Feng Shu" . 
"tumashu@163.com") (:keywords "convenience" "chinese" "pinyin" "input-method") 
(:url . "https://github.com/tumashu/pyim";)) :signed nil) nil ("HEAD"))
  funcall-interactively(package-vc-prepare-patch #s(package-desc :name pyim 
:version (5 2 8) :summary "A Chinese input method support quanpin, 
shuangpin,..." :reqs ((emacs (25 1)) (async (1 6)) (xr (1 13))) :kind vc 
:archive nil :dir "/home/feng/.emacs.d/elpa-29/pyim" :extras ((:commit . 
"93fab3c9e8c2490687781d45c1cecead0bd3ecf2") (:authors ("Ye Wenbin" . 
"wenbinye@163.com") ("Feng Shu" . "tumashu@163.com")) (:maintainer "Feng Shu" . 
"tumashu@163.com") (:keywords "convenience" "chinese" "pinyin" "input-method") 
(:url . "https://github.com/tumashu/pyim";)) :signed nil) nil ("HEAD"))
  call-interactively(package-vc-prepare-patch record nil)
  command-execute(package-vc-prepare-patch record)
  execute-extended-command(nil "package-vc-prepare-patch" nil)
  funcall-interactively(execute-extended-command nil "package-vc-prepare-patch" 
nil)
  call-interactively(execute-extended-command nil nil)
  command-execute(execute-extended-command)

```
--