largefiles: omit redundant isstandin() before splitstandin()
There are many isstandin() invocations before splitstandin().
The former examines whether specified path starts with ".hglf/". The
latter returns after ".hglf/" of specified path if it starts with that
prefix, or returns None otherwise.
Therefore, value returned by splitstandin() can be used for
replacement of preceding isstandin(), and this replacement can omit
redundant string comparison after isstandin().
wrap 0: [0, 'orig']
wrap 1: [1, 0, 'orig']
wrap 2: [2, 1, 0, 'orig']
wrap 3: [3, 2, 1, 0, 'orig']
wrap 4: [4, 3, 2, 1, 0, 'orig']
wrap 0: [0, 4, 3, 2, 1, 0, 'orig']
unwrap 3: 3: [0, 4, 2, 1, 0, 'orig']
unwrap -: 0: [4, 2, 1, 0, 'orig']
unwrap 0: 0: [4, 2, 1, 'orig']
unwrap 4: 4: [2, 1, 'orig']
unwrap 0: -: ValueError
unwrap 2: 2: [1, 'orig']
unwrap 1: 1: ['orig']
unwrap -: -: IndexError