Merge branch 'da/subtree-date-confusion' into maint

"git subtree" (in contrib/) depended on "git log" output to be
stable, which was a no-no.  Apply a workaround to force a
particular date format.

* da/subtree-date-confusion:
contrib/subtree: ignore log.date configuration

diff --cc contrib/subtree/git-subtree.sh
Simple merge

diff --cc contrib/subtree/t/t7900-subtree.sh
index bd3df97936eb77e39642ec917a8d3366945fb0a8,da709f96e57d3395b976968048a12bee005efcb9..90519823be381390b7f89a1eb2f6007f1e8c86f4
--- 1/contrib/subtree/t/t7900-subtree.sh
--- 2/contrib/subtree/t/t7900-subtree.sh
+++ b/contrib/subtree/t/t7900-subtree.sh
@@@ -94,11 -94,15 +94,15 @@@ test_expect_success 'add sub3' 
  # Back to mainline
  cd ..
  
+ test_expect_success 'enable log.date=relative to catch errors' '
+       git config log.date relative
+ '
+ 
  test_expect_success 'add main4' '
 -        create main4 &&
 -        git commit -m "main4" &&
 -        git branch -m master mainline &&
 -        git branch subdir
 +      create main4 &&
 +      git commit -m "main4" &&
 +      git branch -m master mainline &&
 +      git branch subdir
  '
  
  test_expect_success 'fetch subproj history' '