fix make_patchnum.pl branch determination

A previous commit of mine, 9077509,
tried to handle the case where the output of 'git branch' was

    * (no branch)
      blead

to stop it outputting  a 'Use of uninitialized value' warning.
However, my "fix" broke in the case where multiple branches are listed
with the current not the first:

      blead
    * tmp1
      tmp2

Hopefully this fixes it properly.

make_patchnum.pl

@@ -133,7 +133,8 @@ sub write_files {
 }
 elsif (-d "$srcdir/.git") {
     # git branch | awk 'BEGIN{ORS=""} /\*/ { print $2 }'
-    ($branch) = map { /\* ([^(]\S*)/ ? $1 : "" } backtick("git branch");
+    ($branch) = map { /\* ([^(]\S*)/ ? $1 : () } backtick("git branch");
+    $branch //= "";
     my ($remote,$merge);
     if (length $branch) {
         $merge= backtick("git config branch.$branch.merge");