[PATCH]: uniq: add "--group" option

[Assaf Gordon]

Hello,

Attached is a suggestion for "--group" option in uniq, as discussed here:
  http://lists.gnu.org/archive/html/coreutils/2011-03/msg00000.html
  http://lists.gnu.org/archive/html/coreutils/2012-03/msg00052.html

The patch adds two parameters:
      --group=[method]  separate each unique line (whether duplicated or not)
                        with a marker.
                        method={none,separate(default),prepend,append,both)
      --group-separator=SEP   with --group, separates group using SEP
                        (default: empty line)

And it behaves "as expected":
===
$ printf "a\na\na\nb\nc\nc\n" | ./src/uniq --group-sep="--" --group=separate
a
--
b
--
c

$ printf "a\na\na\nb\nc\nc\n" | ./src/uniq --group-sep="--" --group=both
--
a
--
b
--
c
--

$ printf "a\na\na\nb\nc\nc\n" | ./src/uniq --group-sep="--" --group=prepend
--
a
--
b
--
c

$ printf "a\na\na\nb\nc\nc\n" | ./src/uniq -D --group-sep="--" --group=both
--
a
a
a
--
c
c
--

===

The added tests check all sorts of combinations.

If this is the right direction, I'll send an updated patch (with 
NEWS/docs/etc.).

-gordon

0001-uniq-add-group-option.patch
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