Use /bin/sh rather than /bin/bash in test

[MarkEWaite]

Use /bin/sh instead of /bin/bash in test

Fix test on Unix platforms that do not include bash by default.

Tests performed

On FreeBSD and OpenBSD (where /bin/bash is not available), I run

$ git checkout master
$ mvn clean -Dtest=GitHooksTest test
HEAD is now at e98fea9 Jenkinsfile
fatal: cannot run .git/hooks/post-checkout: No such file or directory

Using this pull request, the failing test is resolved:

$ git checkout -b use-posix-shell -t origin/use-posix-shell
$ mvn clean -Dtest=GitHooksTest test
Test passes

Checklist

• I have read the CONTRIBUTING doc
• I have added tests that verify my changes
• Unit tests pass locally with my changes
• I have added documentation as necessary
• No Javadoc warnings were introduced with my changes
• No spotbugs warnings were introduced with my changes
• Documentation in README has been updated as necessary
• Online help has been added and reviewed for any new or modified fields
• I have interactively tested my changes
• Any dependent changes have been merged and published in upstream modules (like git-client-plugin)

Types of changes

• Test