Merge branch 'jc/diff-tree-stdin-fix'

"diff-tree --stdin" has been broken for about a year, but 2.36 release broke it even worse by breaking running the command with <pathspec>, which in turn broke "gitk" and got noticed. This has been corrected by aligning its behaviour to that of "log". * jc/diff-tree-stdin-fix: 2.36 gitk/diff-tree --stdin regression fix