Expose SelectedCommit to custom commands, deprecate Selected{Local,Reflog,Sub}Commit

SelectedCommit is context-dependent and points to SelectedLocalCommit,
SelectedReflogCommit, or SelectedSubCommit depending on which panel is active.

If none of these panels is active, it returns the selected local commit, which
is probably the most useful default (e.g. when defining custom commands for the
Files panel).

docs/Custom_Command_Keybindings.md

@@ -296,9 +296,7 @@ Here's an example using a command but not specifying anything else: so each line
 Your commands can contain placeholder strings using Go's [template syntax](https://jan.newmarch.name/golang/template/chapter-template.html). The template syntax is pretty powerful, letting you do things like conditionals if you want, but for the most part you'll simply want to be accessing the fields on the following objects:
 
 ```
-SelectedLocalCommit
-SelectedReflogCommit
-SelectedSubCommit
+SelectedCommit
 SelectedFile
 SelectedPath
 SelectedLocalBranch
@@ -311,6 +309,9 @@ SelectedWorktree
 CheckedOutBranch
 ```
 
+(For legacy reasons, `SelectedLocalCommit`, `SelectedReflogCommit`, and `SelectedSubCommit` are also available, but they are deprecated.)
+
+
 To see what fields are available on e.g. the `SelectedFile`, see [here](https://github.com/jesseduffield/lazygit/blob/master/pkg/gui/services/custom_commands/models.go) (all the modelling lives in the same file).
 
 ## Keybinding collisions