Support externalDiffCommand in diffing mode (#3519)

- **PR Description**

Support `git.paging.externalDiffCommand` config in diffing mode (i.e.
when showing the diff between two commits using `W`).

Fixes #3518.

pkg/commands/git_commands/diff.go

@@ -1,6 +1,10 @@
 package git_commands
 
-import "github.com/jesseduffield/lazygit/pkg/commands/oscommands"
+import (
+ "fmt"
+
+ "github.com/jesseduffield/lazygit/pkg/commands/oscommands"
+)
 
 type DiffCommands struct {
  *GitCommon
@@ -13,10 +17,16 @@ func NewDiffCommands(gitCommon *GitCommon) *DiffCommands {
 }
 
 func (self *DiffCommands) DiffCmdObj(diffArgs []string) oscommands.ICmdObj {
+ extDiffCmd := self.UserConfig.Git.Paging.ExternalDiffCommand
+ useExtDiff := extDiffCmd != ""
+
  return self.cmd.New(
  NewGitCmd("diff").
  Config("diff.noprefix=false").
- Arg("--submodule", "--no-ext-diff", "--color").
+ ConfigIf(useExtDiff, "diff.external="+extDiffCmd).
+ ArgIfElse(useExtDiff, "--ext-diff", "--no-ext-diff").
+ Arg("--submodule").
+ Arg(fmt.Sprintf("--color=%s", self.UserConfig.Git.Paging.ColorArg)).
  Arg(diffArgs...).
  Dir(self.repoPaths.worktreePath).
  ToArgv(),