Regression in git-subtree.sh, introduced in 2.20.1, after 315a84f9aa0e2e629b0680068646b0032518ebed

Hi

One of the last three commits in git-subtree.sh introduced a regression leading to a segfault.

Here is the error message when I try to split out my i18n files:

$ git subtree split --prefix=i18n
cache for e39a2a0c6431773a5d831eb3cb7f1cd40d0da623 already exists!
   (Lots of output omitted)
436/627 (1819) [1455]       <- Stays at 436/ while the numbers in () and [] increase, then segfaults:
/usr/libexec/git-core/git-subtree: line 751: 54693 Done                    eval "$grl"
    54694 Segmentation fault      (core dumped) | while read rev parents; do
   process_split_commit "$rev" "$parents" 0;
done

Please note that this regression can not easily be reproduced, normally a subtree split just works.

Reverting the last three commits "fixes" the issue.  So I kindly ask the last three commits to be reverted.