t: add greplint to detect bare grep assertions

t: add greplint to detect bare grep assertions

Without a lint guard, bare grep assertions will creep back into
tests over time, defeating the previous commit's conversion.

Add greplint.pl to catch bare 'grep' used as a test assertion
(where 'test_grep' should be used) and '! test_grep' (where
'test_grep !' should be used).

greplint.pl reuses the shared shell parser from lib-shell-parser.pl
to tokenize test bodies.  The Lexer collapses heredocs, command
substitutions, and quoted strings into single tokens, so 'grep'
appearing inside these contexts is not flagged.  A flat walk over
the token stream tracks command position and pipeline state to
distinguish assertion greps from filter greps.

For double-quoted test bodies, a source-line walk counts
backslash-continuation lines that the Lexer consumes without
emitting into the body text, adjusting the reported line number
accordingly.

Add test fixtures in greplint/ (modeled on chainlint/) covering
detection of bare grep assertions, correct skipping of filters,
pipelines, redirects, command substitutions, and lint-ok annotations.

Wire into the Makefile as:
  - test-greplint: runs greplint.pl on $(T) $(THELPERS) $(TPERF)
  - check-greplint: runs greplint.pl on fixtures, diffs against expected
  - clean-greplint: removes temp dir

Add eol=lf entries in t/.gitattributes for greplint fixtures,
matching chainlint, so that check-greplint passes on Windows
where core.autocrlf would otherwise cause CRLF mismatches
between expected and actual output.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>