Code review comment for lp:~parthm/bzr/503670-grep-builtin

=== Fixes #503670 ===
Ports the lp:bzr-grep (https://launchpad.net/bzr-grep) plugin to be a builtin.

[bzrlib]% ../bzr help grep
Purpose: Print lines matching PATTERN for specified files and revisions.
Usage: bzr grep PATTERN [PATH...]

Options:
  --from-root Search for pattern starting from the root of the
                        branch. (implies --recursive)
  -Z, --null Write an ascii NUL (\0) separator between output lines
                        rather than a newline.
  -v, --verbose Display more information.
  -R, --recursive Recurse into subdirectories.
  -h, --help Show help message.
  -q, --quiet Only display errors and warnings.
  -i, --ignore-case ignore case distinctions while matching.
  --levels=N Number of levels to display - 0 for all, 1 for
                        collapsed (default).
  --usage Show usage message and options.
  -n, --line-number show 1-based line number.
  -r ARG, --revision=ARG
                        See "help revisionspec" for details.

Description:
  This command searches the specified files and revisions for a given pattern.
  The pattern is specified as a Python regular expressions[1].
  If the file name is not specified the file revisions in the current directory
  are searched. If the revision number is not specified, the latest revision is
  searched.

  Note that this command is different from POSIX grep in that it searches the
  revisions of the branch and not the working copy. Unversioned files and
  uncommitted changes are not seen.

  When searching a pattern, the output is shown in the 'filepath:string' format.
  If a revision is explicitly searched, the output is shown as 'filepath~N:string',
  where N is the revision number.

  [1] http://docs.python.org/library/re.html#regular-expression-syntax

[bzrlib]%