beinstall: enable use with git worktrees.

svn path=/head/; revision=335831

tools/build/beinstall.sh

@@ -101,7 +101,8 @@ trap 'errx "Interrupt caught"' HUP INT TERM
 objdir=$(make -V .OBJDIR 2>/dev/null)
 [ ! -d "${objdir}" ] && errx "Must have built FreeBSD from source tree"
 
-if [ -d .git ] ; then
+# May be a worktree, in which case .git is a file, not a directory.
+if [ -e .git ] ; then
     commit_time=$(git show --format='%ct' 2>/dev/null | head -1)
     [ $? -ne 0 ] && errx "Can't lookup git commit timestamp"
     commit_ts=$(date -r ${commit_time} '+%Y%m%d.%H%M%S')