Merge branch 'tb/utf-16-le-with-explicit-bom'

A new encoding UTF-16LE-BOM has been invented to force encoding to
UTF-16 with BOM in little endian byte order, which cannot be directly
generated by using iconv.

* tb/utf-16-le-with-explicit-bom:
Support working-tree-encoding "UTF-16LE-BOM"