fix: ‘vc-ediff’ thinks everything in all files has changed

Calling ‘p4-lowlevel-command-or-error’ on a buffer will output the raw response
from Perforce, which includes tags like ‘text:’ and ‘exit:’ at the start of each
line. These tags are used by the low-level functions to find the right data and
figure out if the given command succeeded or not.

Using ‘p4-lowlevel-command-into-buffer’ should have the desired behaviour of
cutting out all the tags and showing only the actual contents of the response.
This will make sure that the “other” buffer doesn't display all lines have
changed because suddenly each line starts with a tag.

p4-lowlevel.el

@@ -626,7 +626,7 @@ then the `-q' flag is passed to `p4 print'."
   (let* ((fullfile (if rev (concat file rev) file))
          (quiet-args (if quiet (list "-q")))
          (args (append (list "print") quiet-args (list fullfile))))
-    (p4-lowlevel-command-or-error args nil output-format)))
+    (p4-lowlevel-command-into-buffer args output-format)))
 
 ;; Here's what we need to support from the "p4 reopen" command, at least for the
 ;; time being: