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

diff --cc log-tree.c
Simple merge