templater: strictly parse leading backslashes of '{' (issue4569) (BC)

Because double backslashes are processed as a string escape sequence, '\\{'
should start the template syntax. On the other hand, r'' disables any sort
of \-escapes, so r'\{' can go either way, never start the template syntax
or always start it. I simply chose the latter, which means r'\{' is the same
as '\\{'.

mercurial/templater.py

@@ -87,8 +87,9 b' def compiletemplate(tmpl, context, strto'
         if n < 0:
             parsed.append((strtoken, tmpl[pos:]))
             break
-        if n > 0 and tmpl[n - 1] == '\\':
-            # escaped
+        bs = (n - pos) - len(tmpl[pos:n].rstrip('\\'))
+        if strtoken == 'string' and bs % 2 == 1:
+            # escaped (e.g. '\{', '\\\{', but not '\\{' nor r'\{')
             parsed.append((strtoken, (tmpl[pos:n - 1] + "{")))
             pos = n + 1
             continue

tests/test-command-template.t

@@ -2273,6 +2273,17 b' Test string escaping:'
   <>\n<]>
   <>\n<
 
+Test leading backslashes:
+
+  $ cd latesttag
+  $ hg log -r 2 -T '\{rev} {files % "\{file}"} {files % r"\{file}"}\n'
+  {rev} {file} \head1
+  $ hg log -r 2 -T '\\{rev} {files % "\\{file}"} {files % r"\\{file}"}\n'
+  \2 \head1 \\head1
+  $ hg log -r 2 -T '\\\{rev} {files % "\\\{file}"} {files % r"\\\{file}"}\n'
+  \{rev} \{file} \\\head1
+  $ cd ..
+
 "string-escape"-ed "\x5c\x786e" becomes r"\x6e" (once) or r"n" (twice)
 
   $ hg log -R a -r 0 --template '{if("1", "\x5c\x786e", "NG")}\n'