tr portability fixes

Specifying character ranges in tr differs between System V
and POSIX. In System V, brackets are required (e.g.,
'[A-Z]'), whereas in POSIX they are not.

We can mostly get around this by just using the bracket form
for both sets, as in:

  tr '[A-Z] '[a-z]'

in which case POSIX interpets this as "'[' becomes '['",
which is OK.

However, this doesn't work with multiple sequences, like:

  # rot13
  tr '[A-Z][a-z]' '[N-Z][A-M][n-z][a-m]'

where the POSIX version does not behave the same as the
System V version. In this case, we must simply enumerate the
sequence.

This patch fixes problematic uses of tr in git scripts and
test scripts in one of three ways:

  - if a single sequence, make sure it uses brackets
  - if multiple sequences, enumerate
  - if extra brackets (e.g., tr '[A]' 'a'), eliminate
    brackets

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4022-diff-rewrite.sh

@@ -8,7 +8,10 @@ test_expect_success setup '
 
 	cat ../../COPYING >test &&
 	git add test &&
-	tr 'a-zA-Z' 'n-za-mN-ZA-M' <../../COPYING >test
+	tr \
+	  "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ" \
+	  "nopqrstuvwxyzabcdefghijklmNOPQRSTUVWXYZABCDEFGHIJKLM" \
+	  <../../COPYING >test
 
 '