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 '\\{'.
'a' in d: True
d['a']: va
'b' in d: True
d['b']: vb
'c' in d: True
d['c']: vc
'd' in d: True
d['d']: vd
'a' in d: False
'b' in d: True
d['b']: vb
'c' in d: True
d['c']: vc
'd' in d: True
d['d']: vd
'e' in d: True
d['e']: ve
'b' in d: True
d['b']: vb2
'c' in d: True
d['c']: vc2
'd' in d: True
d['d']: vd
'e' in d: False
'f' in d: True
d['f']: vf
'b' in d: False
'c' in d: False
'd' in d: False
'e' in d: False
'f' in d: False