fix: Do not exclude executables in git walker

[akshaymankar]

Fixes #388

[brianmcgee]

Thanks for the fix, I was just about to have a look at it myself.

The change makes sense to me, but it has me wondering why the TestGitWorktree in cli/format_test.go wasn't detecting this 🤔 We have a sample shellscript test/examples/shell/foo.sh which is marked as executable. There are 32 files in the example directory, but if we are skipping executable files it should have been detecting 31:

treefmt/cli/format_test.go

Lines 528 to 531 in 1427cc2

	// add everything to the worktree
	as.NoError(wt.AddGlob("."))
	as.NoError(err)
	run(32, 32, 32, 0)

[brianmcgee]

Ok, I understand what was happening and am fixing the test.