April 7 cutover for generating ChangeLog automatically

[Paul Eggert]

Currently, each Emacs commit typically contains two copies of its ChangeLog 
entries: one copy in the commit message and one copy as an edit to one or more 
ChangeLog files.  The latter copy is largely redundant and complicates merging, 
so as discussed in Bug#19113 on April 7 we plan to revamp the Emacs master 
commit procedure to eliminate the second copy, so that ordinary commits do not 
alter ChangeLog files.  Instead, the ChangeLog file in the Emacs distribution 
will be generated automatically from recent Git commit messages.

You'll still be able to edit ChangeLog history by running 'make change-history' 
and then committing changes to a new top-level file (initially 'ChangeLog.1') 
that contains an editable copy of non-recent Git commit messages.  However, 
commits ordinarily shouldn't change ChangeLog files and this should simplify 
merging.

The suggested format for commit messages will be changed slightly:

 - File names should be relative to the top level, e.g.:

        Deactivate shifted region

        Do not silently extend an unhighlighted region;
        this can happen after a shift.
        * doc/emacs/mark.texi (Shift Selection): Document this.
        * lisp/window.el (handle-select-window):
        * src/frame.c (Fhandle_switch_frame, Fselected_frame):
        Deactivate the mark.
        Fixes: bug#19003

   (The actual commit message should not be indented.)

 - If the commit has authors other than yourself, the commit message should 
contain a separate line like the following:

        Co-authored-by: Joe Schmoe <address@hidden>

 - If the commit is a tiny change that is exempt from copyright paperwork, the 
commit message should contain a separate line like the following:

        Copyright-paperwork-exempt: yes

 - If the commit message should not appear in generated ChangeLogs, its first 
line should start with "; ".

With the new approach there are multiple ways to format ChangeLog entries as 
part of the process of creating a commit message:

 - If you use Emacs VC, you can create a top-level ChangeLog file, and update 
it with 'C-x 4 a' file as usual.  Do not register the ChangeLog file under Git; 
instead, use 'C-c C-a' to insert its contents into into your *vc-log* buffer.

 - Alternatively, you can use the vc-dwim command to maintain commit messages. 
 When you create a source directory, run the shell command 
'git-changelog-symlink-init' to create a symbolic link from ChangeLog to 
.git/c/ChangeLog.  Edit this ChangeLog via its symlink with Emacs commands like 
'C-x 4 a', and commit the change using the shell command 'vc-dwim --commit'. 
Type 'vc-dwim --help' for more.

The above will all be documented in the files CONTRIBUTE (for ordinary commits) 
and admin/notes/repo (for commits that edit ChangeLog history files).

This change will be installed on the master branch.  Other branches can adopt 
this change whenever convenient.  The emacs-24 branch can continue to use the 
old procedure, and changes to ChangeLog files in emacs-24 can be discarded by 
hand when merging to master.

Although this new style of maintenance is common in other GNU projects, Emacs 
has its own special needs and most likely there will be glitches after the 
transition.  We will do our best to straighten out these problems as they occur.