templater: handle SyntaxError when parsing ui.logtemplate

Before, Mercurial would crash with a traceback ending with

  SyntaxError: unmatched quotes

if you configured

  [ui]
  logtemplate = {rev}\n

The SyntaxError is now catched and the string is re-parsed without
requiring quotes.

mercurial/cmdutil.py

@@ -910,7 +910,10 b' def show_changeset(ui, repo, opts, buffe'
     if not (tmpl or style):
         tmpl = ui.config('ui', 'logtemplate')
         if tmpl:
-            tmpl = templater.parsestring(tmpl)
+            try:
+                tmpl = templater.parsestring(tmpl)
+            except SyntaxError:
+                tmpl = templater.parsestring(tmpl, quoted=False)
         else:
             style = util.expandpath(ui.config('ui', 'style', ''))
 

tests/test-command-template.t

@@ -45,6 +45,15 b' Second branch starting at nullrev:'
   $ hg mv second fourth
   $ hg commit -m third -d "2020-01-01 10:01"
 
+Quoting for ui.logtemplate
+
+  $ hg tip --config "ui.logtemplate={rev}\n"
+  8
+  $ hg tip --config "ui.logtemplate='{rev}\n'"
+  8
+  $ hg tip --config 'ui.logtemplate="{rev}\n"'
+  8
+
 Make sure user/global hgrc does not affect tests
 
   $ echo '[ui]' > .hg/hgrc