WIP Add config pagerForStaging

Problems:
- sometimes the wrapping is off; I suspect Decolorize is missing some cases
- in the patch building panel, only shows the formatted diff as long as nothing
  has been staged, because then it wants to color the line starts of the
  included lines
- with delta, selection is almost invisible because we are only changing the
  background color, but delta colors the background too (diff-so-fancy doesn't)

docs/Config.md

@@ -280,6 +280,11 @@ git:
     # ydiff -p cat -s --wrap --width={{columnWidth}}
     pager: ""
 
+    # e.g.
+    # delta --dark --paging=never --color-only
+    # diff-so-fancy --patch
+    pagerForStaging: ""
+
     # If true, Lazygit will use whatever pager is specified in `$GIT_PAGER`, `$PAGER`, or your *git config*. If the pager ends with something like ` | less` we will strip that part out, because less doesn't play nice with our rendering approach. If the custom pager uses less under the hood, that will also break rendering (hence the `--paging=never` flag for the `delta` pager).
     useConfig: false