Add line_info() : Open blame or hunk preview depending on context

[WieeRd]

Since lines that can be blamed are mutually exclusive with lines that are inside hunk,
I suggest adding api/command line_info().
If the cursor is on a hunk, it'll call preview_hunk().
If it's not, it'll call blame_line().

local gs = require("gitsigns")

local function line_info(blame_opts)
  local acts = gs.get_actions()
  local func = acts.preview_hunk or acts.blame_line
  if func then func(blame_opts) end
end

I always confused between blame/hunk preview keybind since they do kinda similar thing,
and came up with this simple implementation few days ago. It sure was very useful.

Would be great if this can get included in the plugin itself.
At least as small tip in README if you don't want to add this as a feature.