add magit-decode-git-path

... and use it to undo the following in some places.

(There might be some other places that could benefit from this.  Or
maybe there are event better places to do this instead, to avoid doing
it repeatedly.)

https://github.com/git/git/blob/master/quote.c

Quoting man:git-config "core.quotepath":

The commands that output paths (e.g.  ls-files, diff), when not given
the -z option, will quote "unusual" characters in the pathname by
enclosing the pathname in a double-quote pair and with backslashes the
same way strings in C source code are quoted. If this variable is set
to false, the bytes higher than 0x80 are not quoted but output as
verbatim. Note that double quote, backslash and control characters are
always quoted without -z regardless of the setting of this variable.

Thanks to Nicolas Richard for investigating this issue.  #581, #840.
Merge pull request #850 from tarsius/decode-filename

add magit-decode-git-path