tests: don't hard-code /bin/bash
From Thomas Klausner in pkgsrc.
Differential Revision: https://phab.mercurial-scm.org/D10507
#require black test-repo
Black needs the real USERPROFILE in order to run on Windows
#if msys
$ USERPROFILE="$REALUSERPROFILE"
$ export USERPROFILE
#endif
$ cd $RUNTESTDIR/..
$ black --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`