changeset_templater: backport separator template from formatter

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.)

--- a/mercurial/cmdutil.py	Sat Apr 22 21:50:51 2017 +0900
+++ b/mercurial/cmdutil.py	Sat Jun 17 16:32:20 2017 +0900
@@ -1587,7 +1587,8 @@
         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 @@
         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))

--- a/tests/test-command-template.t	Sat Apr 22 21:50:51 2017 +0900
+++ b/tests/test-command-template.t	Sat Jun 17 16:32:20 2017 +0900
@@ -257,6 +257,20 @@
   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'