Merge branch 'ls/checkout-encoding'

The new "checkout-encoding" attribute can ask Git to convert the
contents to the specified encoding when checking out to the working
tree (and the other way around when checking in).

* ls/checkout-encoding:
  convert: add round trip check based on 'core.checkRoundtripEncoding'
  convert: add tracing for 'working-tree-encoding' attribute
  convert: check for detectable errors in UTF encodings
  convert: add 'working-tree-encoding' attribute
  utf8: add function to detect a missing UTF-16/32 BOM
  utf8: add function to detect prohibited UTF-16/32 BOM
  utf8: teach same_encoding() alternative UTF encoding names
  strbuf: add a case insensitive starts_with()
  strbuf: add xstrdup_toupper()
  strbuf: remove unnecessary NUL assignment in xstrdup_tolower()

diff --cc Documentation/config.txt
Simple merge

diff --cc Documentation/gitattributes.txt
Simple merge

diff --cc config.c
Simple merge

diff --cc convert.c
Simple merge

diff --cc convert.h
Simple merge

diff --cc environment.c
Simple merge

diff --cc git-compat-util.h
Simple merge

diff --cc sha1-file.c
Simple merge

diff --cc strbuf.c
Simple merge

diff --cc strbuf.h
Simple merge

diff --cc utf8.c
Simple merge