Drop leading space, highlight empty lines, highlight whitespace errors

[dandavison]

Fixes #179 cc @phillipwood

• Unless --keep-plus-minus-markers is in effect, drop the first column so that the real code starts in the first column.

• Highlight addition of trailing whitespace as a whitespace error (i.e. git's blank-at-eol). Style determined by new option --whitespace-error-style which defaults to git's color.diff.whitespace.

• Highlight removals and additions of empty lines with background color in the first column if these would not otherwise be visible (i.e. if the applicable style lacks a background color). Style determined by new options --minus-empty-line-marker-style and --plus-empty-line-marker-style.

• Additions of empty lines at end-of-file are not currently highlighted as whitespace errors, but rather as normal addition-of-empty line. This is contra git's blank-at-eof and is a bug to be addressed in subsequent work.

See git documentation: https://git-scm.com/docs/git-config#Documentation/git-config.txt-corewhitespace

[dandavison]

@phillipwood I've rebased over the commit on which you commented earlier but the call to ansi_term_style.paint() here wraps \033[1K\033[1D with a prefix which sets the background color and a SGR RESET suffix. Therefore I believe the end result is to emit the pattern of ANSI sequences that you suggested (modulo the details of the background color):

'\033[41m\033[1K\033[1D\033[m\n'

But do let me know if something doesn't look right!

[phillipwood]

Ah that explains the reset. However I don't think you need ANSI_CSI_CURSOR_BACK_1 though. Looking back at my comments on the issue I must have copied and pasted the wrong line from where I had been playing around in the terminal. It should have been <set background color><ANSI_CLEAR_TO_BOL><ANSI_SGR_RESET><\n> The ANSI_CSI_CUROR_BACK_1 was left over from something else. Sorry for the confusion

[dandavison]

Right, thanks for that. I have removed the ANSI_CSI_CURSOR_BACK_1.