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

"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". This fixes a regression in 2.36 and is slate to go to 2.36.1 source: <xmqq7d7bsu2n.fsf@gitster.g> * jc/diff-tree-stdin-fix: 2.36 gitk/diff-tree --stdin regression fix