Portability of git shell scripts?

Hi list,

I'm trying to compile/test/use git 2.8.2 on AIX 6.1 with no bash available.
/bin/sh is a hard link to /bin/ksh which is a ksh88, a posix shell.
Is this supposed to work?

As an example: make test fails on nearly every t34* test and on tests
which contain rebase.
The installation of bash (and manually changing the shebang to
/bin/bash) "fixes" all rebase test failures. So obviously git-rebase
is not portable at some point.

Does it make any sense to put work into making these scripts portable,
that is, work with posix shells?
And, as last resort, is it possible to configure git use bash in some
or all shell scripts?

Regards,
Armin