fix(gittools): gittree wouldn't run in branchless repos

gittree used to be `gittreeif @` on the theory that every git repo had @
or HEAD.  But a git repo with no branches at all, not even main, has no
HEAD.  Now that we have repos used exclusively for issues, we have to
deal with zero commits, zero branches.

gittools.sh

@@ -1,6 +1,6 @@
 # Source this file to define some useful aliases for working with many git
 # repos at once.
-# Originally from: 
+# Originally from:
 # https://github.com/nedbat/dot/blob/a517603c4969b017d5604418d05babc4a0f323f8/.rc.sh#L126
 
 
@@ -38,7 +38,9 @@ gittreeif() {
     fi
     find . -name .git -type d -prune | while read d; do
         local d=$(dirname "$d")
-        git -C "$d" rev-parse --verify -q "$test_branch" >& /dev/null || continue
+        if [[ "$test_branch" != "" ]]; then
+            git -C "$d" rev-parse --verify -q "$test_branch" >& /dev/null || continue
+        fi
         if [[ $show_dir == true ]]; then
             echo "---- $d ----"
         fi
@@ -54,6 +56,6 @@ gittreeif() {
 }
 
 gittree() {
-    # @ is in every repo, so this runs on all repos
-    gittreeif @ "$@"
+    # Run a command on all git repos.
+    gittreeif "" "$@"
 }