Add "core.eol" config variable

Introduce a new configuration variable, "core.eol", that allows the user
to set which line endings to use for end-of-line-normalized files in the
working directory.  It defaults to "native", which means CRLF on Windows
and LF everywhere else.

Note that "core.autocrlf" overrides core.eol.  This means that

[core]
	autocrlf = true

puts CRLFs in the working directory even if core.eol is set to "lf".

Signed-off-by: Eyvind Bernhardsen <eyvind.bernhardsen@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.c

@@ -40,6 +40,7 @@ const char *editor_program;
 const char *excludes_file;
 enum auto_crlf auto_crlf = AUTO_CRLF_FALSE;
 int read_replace_refs = 1;
+enum eol eol = EOL_UNSET;
 enum safe_crlf safe_crlf = SAFE_CRLF_WARN;
 unsigned whitespace_rule_cfg = WS_DEFAULT_RULE;
 enum branch_track git_branch_track = BRANCH_TRACK_REMOTE;