Move gen_origin from program to data

That way, 'make change-history' needs to change only ChangeLog.2,
instead of having to change two files.
* ChangeLog.2: Add commit info for range that this file covers.
* Makefile.in (new_commit_regexp): New macro.
(change-history-nocommit): Simplify, by putting what used to be
the gen_origin value into the data (ChangeLog.2) rather than
into the program (gitlog-to-emacslog).
* build-aux/gitlog-to-emacslog (gen_origin): Calculate from
the input file (e.g., ChangeLog.2) rather than by having a
constant in the program.  Substitute it into the output.

ChangeLog.2

@@ -5711,6 +5711,9 @@
 	sh
 	git commit -am"[this commit message]"
 
+This file records repository revisions from
+commit 9d56a21e6a696ad19ac65c4b405aeca44785884a (exclusive) to
+commit 325bf192ae281046834884b12705d6c522871b24 (inclusive).
 See ChangeLog.1 for earlier changes.
 
 ;; Local Variables:

Makefile.in

@@ -1116,19 +1116,21 @@ unchanged-history-files:
 	x=$$(git diff-files --name-only $(CHANGELOG_N) $(emacslog)) && \
 	  test -z "$$x"
 
+# Regular expression that matches the newest commit covered by a ChangeLog.
+new_commit_regexp = ^commit [0123456789abcdef]* (inclusive)
+
 # Copy newer commit messages to the start of the ChangeLog history file,
 # and consider them to be older.
 change-history-nocommit: master-branch-is-current unchanged-history-files
 	-rm -f ChangeLog.tmp
 	$(MAKE) ChangeLog CHANGELOG=ChangeLog.tmp
-	(sed '/^See ChangeLog.[0-9]* for earlier/,$$d' <ChangeLog.tmp && cat $(CHANGELOG_N)) \
-	  >$(CHANGELOG_N).tmp
+	sed '/^This file records repository revisions/,$$d' \
+	  ChangeLog.tmp >$(CHANGELOG_N).tmp
+	new_commit_line=`grep '$(new_commit_regexp)' ChangeLog.tmp` && \
+	sed 's/$(new_commit_regexp).*/'"$$new_commit_line/" \
+	  $(CHANGELOG_N) >>$(CHANGELOG_N).tmp
 	rm ChangeLog.tmp
-	new_origin=$$(git log --pretty=format:%H HEAD^!) && \
-	sed 's/^\(gen_origin=\).*/\1'"$$new_origin/" \
-	  < $(emacslog) > $(emacslog).tmp && chmod +x $(emacslog).tmp
 	mv $(CHANGELOG_N).tmp $(CHANGELOG_N)
-	mv $(emacslog).tmp $(emacslog)
 
 change-history: change-history-nocommit
 	$(MAKE) $@-commit

build-aux/gitlog-to-emacslog

@@ -23,7 +23,8 @@ LC_ALL=C
 export LC_ALL
 
 # The newest revision that should not appear in the generated ChangeLog.
-gen_origin=325bf192ae281046834884b12705d6c522871b24
+gen_origin=
+
 force=
 output=ChangeLog
 nmax=2
@@ -45,6 +46,22 @@ if [ ! -f ChangeLog.$nmax ]; then
     exit 1
 fi
 
+# If not specified in the command line, get gen_origin from the existing
+# ChangeLog file.
+[ "$gen_origin" ] || {
+    gen_origin_line=`
+      grep -E '^commit [0-9a-f]+ [(]inclusive[)]' ChangeLog.$nmax
+    ` || {
+	echo "ChangeLog.$nmax lacks a 'commit ... (inclusive)' line" >&2
+	exit 1
+    }
+    set $gen_origin_line
+    gen_origin=$2
+}
+
+# Get the new value for gen_origin from the latest version in the repository.
+new_origin=`git log --pretty=format:%H 'HEAD^!'` || exit
+
 if [ -f "$output" ]; then
     [ ! "$force" ] && echo "$output exists" >&2 && exit 1
     rm -f "$output" || exit 1
@@ -59,7 +76,7 @@ test -d .git || {
 # Use Gnulib's packaged ChangeLog generator.
 ./build-aux/gitlog-to-changelog --ignore-matching='^; ' \
   --ignore-line='^; ' --format='%B' \
-  "$gen_origin.." >"ChangeLog.tmp" || exit
+  "$gen_origin..$new_origin" >"ChangeLog.tmp" || exit
 
 if test -s "ChangeLog.tmp"; then
 
@@ -90,10 +107,13 @@ if test -s "ChangeLog.tmp"; then
     year_range=$start_year-$end_year
   fi
 
-  # Append a proper copyright notice.
+  # Update gen_origin and append a proper copyright notice.
   sed -n '
     1i\
 
+    /^This file records repository revisions/p
+    s/^commit [0-9a-f]* (exclusive)/commit '"$gen_origin"' (exclusive)/p
+    s/^commit [0-9a-f]* (inclusive)/commit '"$new_origin"' (inclusive)/p
     /^See ChangeLog.[0-9]* for earlier/,${
        s/ChangeLog\.[0-9]*/ChangeLog.'$nmax'/
        s/\(Copyright[ (C)]*\)[0-9]*-[0-9]*/\1'"$year_range"'/