Add a user config for using git's external diff command for paging

This is similar to using lazygit's Git.Paging.ExternalDiffCommand config, except
that the command is configured in git. This can be done either with git's
`diff.external` config, or through .gitattributes, so it gives a bit more
flexibility.

pkg/commands/git_commands/stash_test.go

@@ -104,6 +104,7 @@ func TestStashStashEntryCmdObj(t *testing.T) {
 		similarityThreshold int
 		ignoreWhitespace    bool
 		extDiffCmd          string
+		useExtDiffGitConfig bool
 		expected            []string
 	}
 
@@ -141,6 +142,15 @@ func TestStashStashEntryCmdObj(t *testing.T) {
 			extDiffCmd:          "difft --color=always",
 			expected:            []string{"git", "-C", "/path/to/worktree", "-c", "diff.external=difft --color=always", "stash", "show", "-p", "--stat", "-u", "--ext-diff", "--color=always", "--unified=3", "--find-renames=50%", "refs/stash@{5}"},
 		},
+		{
+			testName:            "Show diff using git's external diff config",
+			index:               5,
+			contextSize:         3,
+			similarityThreshold: 50,
+			ignoreWhitespace:    false,
+			useExtDiffGitConfig: true,
+			expected:            []string{"git", "-C", "/path/to/worktree", "stash", "show", "-p", "--stat", "-u", "--ext-diff", "--color=always", "--unified=3", "--find-renames=50%", "refs/stash@{5}"},
+		},
 		{
 			testName:            "Default case",
 			index:               5,
@@ -158,6 +168,7 @@ func TestStashStashEntryCmdObj(t *testing.T) {
 			userConfig.Git.DiffContextSize = s.contextSize
 			userConfig.Git.RenameSimilarityThreshold = s.similarityThreshold
 			userConfig.Git.Paging.ExternalDiffCommand = s.extDiffCmd
+			userConfig.Git.Paging.UseExternalDiffGitConfig = s.useExtDiffGitConfig
 			repoPaths := RepoPaths{
 				worktreePath: "/path/to/worktree",
 			}