bug#25410: 26.0.50; Refine an unified diff hunk only if adds lines

[Tino Calancha <tino.calancha@gmail.com>]

After deletion of a large file from CVS, a diff shows
a very large hunk with just deleted lines.  Then, for unified diffs, a call
to `diff-refine-hunk' on that hunk takes a huge time.
Instead, it's better to first check if the hunk adds new lines: only when
this is true, then proceed with the hunk refinement.

emacs -Q
M-! git diff ef8c9f8^ ef8c9f8 RET
C-x o
M-x diff-mode RET
C-c C-b

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From b3252092f8fdfc03c02d23022d901a625d183d89 Mon Sep 17 00:00:00 2001
From: Tino Calancha <tino.calancha@gmail.com>
Date: Tue, 10 Jan 2017 18:46:00 +0900
Subject: [PATCH] Refine an unified diff hunk only if adds lines

* lisp/vc/diff-mode.el (diff-refine-hunk): Refine the hunk
only when adds some new lines (Bug#25410).
---
 lisp/vc/diff-mode.el | 25 +++++++++++++------------
 1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index 9dfcd944bb..e045a5d974 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -2075,22 +2075,23 @@ diff-refine-hunk
            (props-c '((diff-mode . fine) (face diff-refine-changed)))
            (props-r '((diff-mode . fine) (face diff-refine-removed)))
            (props-a '((diff-mode . fine) (face diff-refine-added))))
-
       (remove-overlays beg end 'diff-mode 'fine)
-
       (goto-char beg)
       (pcase style
         (`unified
-         (while (re-search-forward
-                 (eval-when-compile
-                   (let ((no-LF-at-eol-re "\\(?:\\\\.*\n\\)?"))
-                     (concat "^\\(?:-.*\n\\)+" no-LF-at-eol-re
-                             "\\(\\)"
-                             "\\(?:\\+.*\n\\)+" no-LF-at-eol-re)))
-                 end t)
-           (smerge-refine-subst (match-beginning 0) (match-end 1)
-                                (match-end 1) (match-end 0)
-                                nil 'diff-refine-preproc props-r props-a)))
+         ;; Refine hunk only when it adds lines (Bug#25410).
+         (when (re-search-forward "^\\(?:\\+.*\n\\)+" end t)
+           (goto-char beg)
+           (while (re-search-forward
+                   (eval-when-compile
+                     (let ((no-LF-at-eol-re "\\(?:\\\\.*\n\\)?"))
+                       (concat "^\\(?:-.*\n\\)+" no-LF-at-eol-re
+                               "\\(\\)"
+                               "\\(?:\\+.*\n\\)+" no-LF-at-eol-re)))
+                   end t)
+             (smerge-refine-subst (match-beginning 0) (match-end 1)
+                                  (match-end 1) (match-end 0)
+                                  nil 'diff-refine-preproc props-r props-a))))
         (`context
          (let* ((middle (save-excursion (re-search-forward "^---")))
                 (other middle))
-- 
2.11.0

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In GNU Emacs 26.0.50.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.5)
 of 2017-01-09
Repository revision: ef8c9f8fc922b615aca91b47820d1f1900fddc96