changeset_templater: backport separator template from formatter

As commented, this should be used with docheader and docfooter, not with
header nor footer. That's one reason why no props are passed to templater
when rendering a separator. (See map-cmdline.changelog to understand what
the "header" is.)

mercurial/cmdutil.py

@@ -1587,7 +1587,8 b' class changeset_templater(changeset_prin'
         self._tref = tmplspec.ref
         self._parts = {'header': '', 'footer': '',
                        tmplspec.ref: tmplspec.ref,
-                       'docheader': '', 'docfooter': ''}
+                       'docheader': '', 'docfooter': '',
+                       'separator': ''}
         if tmplspec.mapfile:
             # find correct templates for current mode, for backward
             # compatibility with 'log -v/-q/--debug' using a mapfile
@@ -1621,11 +1622,17 b' class changeset_templater(changeset_prin'
         props['ctx'] = ctx
         props['repo'] = self.repo
         props['ui'] = self.repo.ui
-        props['index'] = next(self._counter)
+        props['index'] = index = next(self._counter)
         props['revcache'] = {'copies': copies}
         props['cache'] = self.cache
         props = pycompat.strkwargs(props)
 
+        # write separator, which wouldn't work well with the header part below
+        # since there's inherently a conflict between header (across items) and
+        # separator (per item)
+        if self._parts['separator'] and index > 0:
+            self.ui.write(templater.stringify(self.t(self._parts['separator'])))
+
         # write header
         if self._parts['header']:
             h = templater.stringify(self.t(self._parts['header'], **props))

tests/test-command-template.t

@@ -257,6 +257,20 b' Test template map inheritance'
   summary:     third
   
 
+Test docheader, docfooter and separator in template map
+
+  $ cat <<'EOF' > map-myjson
+  > docheader = '\{\n'
+  > docfooter = '\n}\n'
+  > separator = ',\n'
+  > changeset = ' {dict(rev, node|short)|json}'
+  > EOF
+  $ hg log -l2 -T./map-myjson
+  {
+   {"node": "95c24699272e", "rev": 8},
+   {"node": "29114dbae42b", "rev": 7}
+  }
+
 Template should precede style option
 
   $ hg log -l1 --style default -T '{rev}\n'