Improve :BCommits Preview (#1525)

Reduce the preview of :BCommits to the relevant part, when used with visual selection.
Only show the part of the diff that belongs to the selection.

That makes it very easy to quickly follow the evolution of a code block.

autoload/fzf/vim.vim

@@ -1474,14 +1474,22 @@ function! s:commits(range, buffer_local, args)
 
  let args = copy(a:args)
  let log_opts = len(args) && type(args[0]) == type('') ? remove(args, 0) : ''
+ let with_preview = !s:is_win && &columns > s:wide
 
  if len(a:range) || a:buffer_local
  if !managed
  return s:warn('The current buffer is not in the working tree')
  endif
- let source .= len(a:range)
- \ ? join([printf(' -L %d,%d:%s --no-patch', a:range[0], a:range[1], fzf#shellescape(current)), log_opts])
- \ : join([' --follow', log_opts, fzf#shellescape(current)])
+ if len(a:range)
+ let source .= join([printf(' -L %d,%d:%s --no-patch', a:range[0], a:range[1], fzf#shellescape(current)), log_opts])
+ if with_preview
+ let previewparams = join([printf('log -L %d,%d:%s', a:range[0], a:range[1], fzf#shellescape(current)), log_opts])
+ let previewfilter = " | awk '/commit {1}/ {flag=1;print;next} /^[^ ]*commit/{flag=0} flag' "
+ let previewcmd = prefix . previewparams .' --color=always '. previewfilter
+ endif
+ else
+ let source .= join([' --follow', log_opts, fzf#shellescape(current)])
+ endif
  let command = 'BCommits'
  else
  let source .= join([' --graph', log_opts])
@@ -1503,12 +1511,14 @@ function! s:commits(range, buffer_local, args)
  let options.options[-1] .= ',ctrl-d'
  endif
 
- if !s:is_win && &columns > s:wide
+ if with_preview
+ if !len(a:range)
+ let orderfile = tempname()
+ call writefile([current[len(s:git_root)+1:]], orderfile)
+ let previewcmd = 'echo {} | grep -o "[a-f0-9]\{7,\}" | head -1 | xargs ' . prefix . 'show -O'.fzf#shellescape(orderfile).' --format=format: --color=always '
+ endif
  let suffix = executable('delta') ? '| delta --width $FZF_PREVIEW_COLUMNS' : ''
- let orderfile = tempname()
- call writefile([current[len(s:git_root)+1:]], orderfile)
- call extend(options.options,
- \ ['--preview', 'echo {} | grep -o "[a-f0-9]\{7,\}" | head -1 | xargs ' . prefix . 'show -O'.fzf#shellescape(orderfile).' --format=format: --color=always ' . suffix])
+ call extend(options.options, ['--preview', previewcmd . suffix])
  endif
 
  return s:fzf(a:buffer_local ? 'bcommits' : 'commits', options, args)