[GitWorktree#checkout_at] Pass current commit_sha
[GitWorktree#checkout_at] Pass current commit_sha

This fixes a bug where if a user specified a particular branch, it would
not be respected by the system, and would always checkout origin/master
for the parent repo.

This was not a problem for submodules, since the `@commit_sha` was
already being passed in for the clone of those repos.

gmcculloug/manageiq,ManageIQ/manageiq,ManageIQ/manageiq,jrafanie/manageiq,agrare/manageiq,hstastna/manageiq,billfitzgerald0120/manageiq,lpichler/manageiq,jrafanie/manageiq,d-m-u/manageiq,gmcculloug/manageiq,mzazrivec/manageiq,billfitzgerald0120/manageiq,mzazrivec/manageiq,skateman/manageiq,d-m-u/manageiq,jrafanie/manageiq,djberg96/manageiq,djberg96/manageiq,lpichler/manageiq,kbrock/manageiq,agrare/manageiq,chessbyte/manageiq,kbrock/manageiq,jrafanie/manageiq,mzazrivec/manageiq,gmcculloug/manageiq,chessbyte/manageiq,jvlcek/manageiq,jvlcek/manageiq,jvlcek/manageiq,hstastna/manageiq,billfitzgerald0120/manageiq,agrare/manageiq,skateman/manageiq,kbrock/manageiq,lpichler/manageiq,NickLaMuro/manageiq,ManageIQ/manageiq,billfitzgerald0120/manageiq,chessbyte/manageiq,hstastna/manageiq,djberg96/manageiq,agrare/manageiq,chessbyte/manageiq,kbrock/manageiq,lpichler/manageiq,mzazrivec/manageiq,djberg96/manageiq,NickLaMuro/manageiq,gmcculloug/manageiq,hstastna/manageiq,ManageIQ/manageiq,skateman/manageiq,jvlcek/manageiq,d-m-u/manageiq,skateman/manageiq,d-m-u/manageiq,NickLaMuro/manageiq,NickLaMuro/manageiq