disable vc-dir-process-output-limit for diff-hl-dired - #294

Recent upstream Emacs change (268e055e051 (“Limit VC-Dir status process output processing”)) introduced vc-dir-process-output-limit variable with default value of 3000 characters & changed VC backend dir-status-files implementations to call vc-dir-maybe-narrow-and-show-more-button before processing larger output.

Basically if there are changes in excess of limit, emacs truncates the result and adds a button to rerun the refresh without a limit, introducing dependency on status process buffer’s vc-parent-buffer, pointing to the VC-Dir buffer, which is not present when diff-hl ends up calling that code.

This breaks diff-hl in dired, producing an error like this:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  vc-dir-maybe-narrow-and-show-more-button("(reported states may be incorrect)")
  #f(compiled-function () #<bytecode -0x1355c4af29e0d36f>)()
  #f(compiled-function () #<bytecode 0xe6ea5554b3be332>)()
  #f(compiled-function (proc msg) #<bytecode 0x6d883a17e9c9bdb>)(#<process git> "finished\n")
  apply(#f(compiled-function (proc msg) #<bytecode 0x6d883a17e9c9bdb>) (#<process git> "finished\n"))
  #f(advice #f(compiled-function (proc msg) #<bytecode 0x6d883a17e9c9bdb>) :after #f(advice #f(compiled-function (proc msg) #<bytecode 0x6d883a17e9c9bdb>) :after ignore))(#<process git> "finished\n")

Proposed change is to not have that output limit in diff-hl’s private status buffer.