Support core.autocrlf = input

Support core.autocrlf = input

The core.autocrlf variable can take on three values: false, true,
and input.  Parsing it as a boolean is wrong, we instead need to
parse a tri-state enumeration.

Add support for parsing and setting enum values from Java from and
to the text based configuration file, and use that to handle the
autocrlf variable.

Bug: 301775
Change-Id: I81b9e33087a33d2ef2eac89ba93b9e83b7ecc223
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>