Fix handling of quoted strings in the config file

Fix handling of quoted strings in the config file

parse_string() adapts the string length to automatically remove quotation
marks when copying the string. However, when calling string_ncopy_do()
strlen(arg) used to be called again instead of using the adapted value.

This e.g. led to wrong locale settings when using
  set commit-encoding = "UTF-8"
and thus a slightly messed up display.

Thanks to Gerfried Fuchs for reporting this.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Jonas Fonseca <fonseca@diku.dk>