bug#25105: bug#25400: M-p in diff-mode jumps too far

[Tino Calancha]

On Fri, 13 Jan 2017, Dmitry Gutov wrote:

> I wish the first one could be produced with a simple 'git revert', but that 
> leads to a conflict.
Sorry for the confusin.  I made the revert manually.
In addition to commit 2c8a7e5,  i've reverted following
other related commits:

e5ef59b87da5c2ddfa22f7342efe29b3eea6ed97
6b6abe0dba6a9a2e5f78aac3814421886e7a184f
73349822cbd6e50526eda9c75453584d73dfca83
a283d655db88cdcc8cb53d8e2578e1cdf751c84b
61c6a10e3110490dadac4577cc540053341ff25c
2c8a7e50d24daf19ea7d86f1cfeaa98a41c56085


In scratch/calancha-revert-2c8a7e5 branch,
diff-mode.el history is as follows:

##### _before_ my changes #####
;; initial state for diff-mode.el is as in commit:
1f5592572887fe15e5b660bc60e66a7ab7c624cd ; w/ copyright year 2016
;; Update copyright year and we are at 
;; 3e30cda89474209716c6e16a1a81d02877c95a2b
;; i.e., first commit in my new branch).

;; then, second commit (0beb7d2968ab76878eb3be26f2d749977fdcaa2f)
;; add my fix for Bug#17544.