getting emacs-25-to-master merges working again

[Paul Eggert]

Today I got burned again by a bug in Emacs master that has been fixed in 
emacs-25.  Right now, my impression is that the master branch has serious 
unfixed bugs; I wouldn't recommend it for routine use. At some point we need to 
get emacs-25 merged with master, to fix the bugs. I drafted a patch to help do 
this (attached). It still needs work but I thought I'd send what I have, to help 
get the ball rolling.

The main problem in getting merging to work again is that changes to ChangeLog.2 
in emacs-25 will collide with changes to ChangeLog.2 in the master. The attached 
patch attempts to fix this by renaming ChangeLog.2 to ChangeLog-25.0 in 
emacs-25, and to create a new file ChangeLog-25.1 in master, such that 'make 
change-history' updates ChangeLog-25.0 in emacs-25 and updates ChangeLog-25.1 in 
master.  That way, the two sets of histories ordinarily shouldn't overlap and 
changes to one shouldn't collide with the other.

Undoubtedly there will be kinks in this process that need to be ironed out, 
e.g., when backporting changes from master to emacs-25. One way to iron them out 
will be to review and/or improve this patch. Another is to install this patch, 
do a merge, and deal with the resulting fallout.  Either will take nontrivial work.

I'm thinking, though, that there should be a better way, by using the --cherry 
option of 'git log', and by generating ChangeLog.2 and/or ChangeLog-25.1 
specially during a merge. I haven't had time to investigate this yet, though.

emacs-25-merge.patch
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