templater: fix escaping in nested string literals (issue4102)

Before the templater got extended for nested expressions, it made
sense to decode string escapes across the whole string. Now we do it
on a piece by piece basis.

mercurial/cmdutil.py

@@ -940,9 +940,7 b' def show_changeset(ui, repo, opts, buffe'
 
     tmpl = opts.get('template')
     style = None
-    if tmpl:
-        tmpl = templater.parsestring(tmpl, quoted=False)
-    else:
+    if not tmpl:
         style = opts.get('style')
 
     # ui settings

mercurial/templater.py

@@ -52,7 +52,7 b' def tokenizer(data):'
                     if not decode:
                         yield ('string', program[s:pos].replace('\\', r'\\'), s)
                         break
-                    yield ('string', program[s:pos].decode('string-escape'), s)
+                    yield ('string', program[s:pos], s)
                     break
                 pos += 1
             else:
@@ -80,19 +80,19 b' def compiletemplate(tmpl, context):'
     parsed = []
     pos, stop = 0, len(tmpl)
     p = parser.parser(tokenizer, elements)
-
     while pos < stop:
         n = tmpl.find('{', pos)
         if n < 0:
-            parsed.append(("string", tmpl[pos:]))
+            parsed.append(("string", tmpl[pos:].decode("string-escape")))
             break
         if n > 0 and tmpl[n - 1] == '\\':
             # escaped
-            parsed.append(("string", tmpl[pos:n - 1] + "{"))
+            parsed.append(("string",
+                           (tmpl[pos:n - 1] + "{").decode("string-escape")))
             pos = n + 1
             continue
         if n > pos:
-            parsed.append(("string", tmpl[pos:n]))
+            parsed.append(("string", tmpl[pos:n].decode("string-escape")))
 
         pd = [tmpl, n + 1, stop]
         parseres, pos = p.parse(pd)

mercurial/templates/paper/shortlog.tmpl

@@ -80,8 +80,8 b''
                 return m ? m[1] : null;
             },
             '.bigtable > tbody:nth-of-type(2)',
-            '<tr class="%class%">\
-            <td colspan="3" style="text-align: center;">%text%</td>\
+            '<tr class="%class%">\\
+            <td colspan="3" style="text-align: center;">%text%</td>\\
             </tr>'
     );
 </script>

tests/test-command-template.t

@@ -1586,3 +1586,11 b' Test the strip function with chars speci'
   h1c
   b
   a
+
+Test string escaping:
+
+  $ hg log -R latesttag -r 0 --template '>\n<>\\n<{if(rev, "[>\n<>\\n<]")}>\n<>\\n<\n'
+  >
+  <>\n<[>
+  <>\n<]>
+  <>\n<