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()

environment.c

@@ -55,6 +55,7 @@ int check_replace_refs = 1; /* NEEDSWORK: rename to read_replace_refs */
 char *git_replace_ref_base;
 enum eol core_eol = EOL_UNSET;
 int global_conv_flags_eol = CONV_EOL_RNDTRP_WARN;
+char *check_roundtrip_encoding = "SHIFT-JIS";
 unsigned whitespace_rule_cfg = WS_DEFAULT_RULE;
 enum branch_track git_branch_track = BRANCH_TRACK_REMOTE;
 enum rebase_setup_type autorebase = AUTOREBASE_NEVER;