patchmergetools/vimdiff: add vimdiff1 merge tool variant

> This adds yet another vimdiff/gvimdiff variant and presents conflicts as
> a two-way diff between 'LOCAL' and 'REMOTE'. 'MERGED' is not opened
> which deviates from the norm so usage text is echoed as a Vim message on
> startup that instructs the user with how to proceed and how to abort.
>
> Vimdiff is well-suited to two-way diffs so this is an option for a more
> simple, more streamlined conflict resolution. For example: it is
> difficult to communicate differences across more than two files using
> only syntax highlighting; default vimdiff commands to get and put
> changes between buffers do not need the user to manually specify
> a source or destination buffer when only using two buffers.
>
> Like other merge tools that directly compare 'LOCAL' with 'REMOTE', this
> tool will benefit when paired with the new `mergetool.hideResolved`
> setting.
>
> Signed-off-by: Seth House <seth@eseth.com>
> ---
>  mergetools/vimdiff | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)

> diff --git a/mergetools/vimdiff b/mergetools/vimdiff
> index abc8ce4ec4..96f6209a04 100644
> --- a/mergetools/vimdiff
> +++ b/mergetools/vimdiff
> @@ -15,6 +15,17 @@ merge_cmd () {
>  				"$LOCAL" "$MERGED" "$REMOTE"
>  		fi
>  		;;
> +	*vimdiff1)
> +		"$merge_tool_path" -f -d \
> +			-c 'echon "Resolve conflicts leftward then save. Use :cq to abort."' \
> +			"$LOCAL" "$REMOTE"
> +		ret="$?"
> +		if test "$ret" -eq 0
> +		then
> +			cp -- "$LOCAL" "$MERGED"
> +		fi
> +		return "$ret"
> +		;;
>  	*vimdiff2)
>  		"$merge_tool_path" -f -d -c 'wincmd l' \
>  			"$LOCAL" "$MERGED" "$REMOTE"
> @@ -52,7 +63,7 @@ exit_code_trustable () {
>  
>  list_tool_variants () {
>  	for prefix in '' g n; do
> -		for suffix in '' 2 3; do
> +		for suffix in '' 1 2 3; do
>  			echo "${prefix}vimdiff${suffix}"
>  		done
>  	done

>
> Seth House <seth@eseth.com> writes:
>
> > This adds yet another vimdiff/gvimdiff variant and presents conflicts as
> > a two-way diff between 'LOCAL' and 'REMOTE'. 'MERGED' is not opened
> > which deviates from the norm so usage text is echoed as a Vim message on
> > startup that instructs the user with how to proceed and how to abort.
> >
> > Vimdiff is well-suited to two-way diffs so this is an option for a more
> > simple, more streamlined conflict resolution. For example: it is
> > difficult to communicate differences across more than two files using
> > only syntax highlighting; default vimdiff commands to get and put
> > changes between buffers do not need the user to manually specify
> > a source or destination buffer when only using two buffers.
> >
> > Like other merge tools that directly compare 'LOCAL' with 'REMOTE', this
> > tool will benefit when paired with the new `mergetool.hideResolved`
> > setting.
> >
> > Signed-off-by: Seth House <seth@eseth.com>
> > ---
> >  mergetools/vimdiff | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
>
> Any vimdiff$n + "git mergetool" users care to comment?  Thanks.

> Add yet another vimdiff layout variant to present the simplest possible
> two-way diff when resolving conflicts.
> 
> The name is an attempt at UNIX-style humor -- vimdiff3 opens four
> buffers and vimdiff2 opens three buffers so vimdiff1 should therefore
> open two buffers. It also communicates that vimdiff will be used and
> fits cleanly into the existing layout variant naming. That said,
> suggestions welcome if that isn't as great a fit as I think it is. :)

> I think it would be good to add a short note about these variants somewhere
> in Documentation/git-mergetool.txt. Might be in this patch, or not if you don't
> feel like it...
> 
> Going further, we could even add a short (few words) description of each tool
> and have that description show up in the output of  'git mergetool --tool-help',
> something like this:
> 
> $ git mergetool --tool-help
> 'git mergetool --tool=<tool>' may be set to one of the following:
> 		emerge		Emacs (Emerge)
> 		opendiff	Apple FileMerge
> 		vimdiff		Vim (??)
> 		vimdiff2	Vim (3 panes)
> 		vimdiff3	Vim (4 panes)

> On Thu, Feb 25, 2021 at 01:45:16PM -0500, Philippe Blain wrote:
>> I think it would be good to add a short note about these variants somewhere
>> in Documentation/git-mergetool.txt. Might be in this patch, or not if you don't
>> feel like it...
>> 
>> Going further, we could even add a short (few words) description of each tool
>> and have that description show up in the output of  'git mergetool --tool-help',
>> something like this:
>> 
>> $ git mergetool --tool-help
>> 'git mergetool --tool=<tool>' may be set to one of the following:
>> 		emerge		Emacs (Emerge)
>> 		opendiff	Apple FileMerge
>> 		vimdiff		Vim (??)
>> 		vimdiff2	Vim (3 panes)
>> 		vimdiff3	Vim (4 panes)
>
> Great suggestions, thanks. A little explanation would be very helpful --
> I've been confused by those variants too and wondered why I might want
> to use one over another.
>
> I'll roll those into this patch.

> On Thu, Feb 25, 2021 at 06:04:53PM -0800, Junio C Hamano wrote:
>> Seth House <seth@eseth.com> writes:
>> > I'll roll those into this patch.
>> 
>> I'd rather see it as a completely separate patch.
>
> Ok, will do.
>
> Should I roll a v2 of this patch set to include David's tested-by tag
> even though there's no code changes?