convert: add 'working-tree-encoding' attribute

convert: add 'working-tree-encoding' attribute

Git recognizes files encoded with ASCII or one of its supersets (e.g.
UTF-8 or ISO-8859-1) as text files. All other encodings are usually
interpreted as binary and consequently built-in Git text processing
tools (e.g. 'git diff') as well as most Git web front ends do not
visualize the content.

Add an attribute to tell Git what encoding the user has defined for a
given file. If the content is added to the index, then Git reencodes
the content to a canonical UTF-8 representation. On checkout Git will
reverse this operation.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>