43617: vcs_info git: During a non-interactive rebase of a detached head, computer the %b expando correctly.

Before this commit, the value of %b was the hash of the commit from the
"source" side of the rebase, from .git/rebase-apply/orig-head and
.git/rebase-apply/original-commit.  This broke the invariant that
%b expands to a git-rev-parse(1) expression resolving to what %r
expands to.

Use .git/rebase-apply/onto instead as, empirically, it contains the
correct value.

Functions/VCS_Info/Backends/VCS_INFO_get_data_git

@@ -90,7 +90,7 @@ VCS_INFO_git_getbranch () {
         [[ -z ${gitbranch} ]] && [[ -r ${actiondir}/head-name ]] \
             && gitbranch="$(< ${actiondir}/head-name)"
         [[ -z ${gitbranch} || ${gitbranch} == 'detached HEAD' ]] \
-            && gitbranch="$(< ${gitdir}/ORIG_HEAD)"
+            && gitbranch="$(< ${actiondir}/onto)"
 
     elif [[ -f "${gitdir}/MERGE_HEAD" ]] ; then
         gitbranch="$(${(z)gitsymref} 2> /dev/null)"