t0025: rename the test files

The current test files are named one, two and three.
Make it clearer what the tests do and rename them into
LFonly, CRLFonly and LFwithNUL.

After the renaming we can see easier that we may want more test cases
for 2 types of files:
- files which have mixed LF and CRLF line endings,
- files which have mixed LF and CR line endings.

See commit fd6cce9e, "Add per-repository eol normalization" and
"the new safer autocrlf handling" in convert.c

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
t0027: combinations of core.autocrlf, core.eol and text

Historically there are 3 different parameters controlling how line endings
are handled by Git:
- core.autocrlf
- core.eol
- the "text" attribute in .gitattributes

There are different types of content:
- (1) Files with only LF
- (2) Files with only CRLF
- (3) Files with mixed LF and CRLF
- (4) Files with LF and/or CRLF with CR not followed by LF
- (5) Files which are binary (e.g. have NUL bytes)

Recently the question came up, how files with mixed EOLs are handled by Git
(and libgit2) when they are checked out and core.autocrlf=true.

See
http://git.661346.n2.nabble.com/The-different-EOL-behavior-between-libgit2-based-software-and-official-Git-td7613670.html#a7613801

Add the EXPENSIVE t0027-auto-crlf.sh to test all combination of files
and parameters for both "git add/commit" and "git checkout".

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>