[...]

do so (now or later) by using -b with the checkout command again. Example:
  git checkout -b new_branch_name
HEAD is now at 2a55bfc... - verbose files, age note (it's almost 18y old...) - adjusted info patch
fatal: HEAD does not point to a branch
Available branches: master
Searching for tag auto/ti/autoconf2_13-2.13-1...
Searching for tag auto/ti/autoconf2_13-2_13-1...
--2018-04-14 19:06:03--  http://distfiles.pld-linux.org/distfiles/by-md5/9/d/9de56d4a161a723228220b0f425dc711/autoconf-2.13.tar.gz
Resolving distfiles.pld-linux.org (distfiles.pld-linux.org)... 193.239.45.41
Connecting to distfiles.pld-linux.org (distfiles.pld-linux.org)|193.239.45.41|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 443844 (433K) [application/x-gzip]
Saving to: 'autoconf-2.13.tar.gz'
     0K .......... .......... .......... .......... .......... 11% 1.21M 0s
    50K .......... .......... .......... .......... .......... 23% 2.37M 0s
   100K .......... .......... .......... .......... .......... 34% 60.3M 0s
   150K .......... .......... .......... .......... .......... 46% 98.8M 0s
   200K .......... .......... .......... .......... .......... 57% 2.43M 0s
   250K .......... .......... .......... .......... .......... 69%  110M 0s
   300K .......... .......... .......... .......... .......... 80%  239M 0s
   350K .......... .......... .......... .......... .......... 92%  240M 0s
   400K .......... .......... .......... ...                  100%  238M=0.08s
2018-04-14 19:06:03 (5.08 MB/s) - 'autoconf-2.13.tar.gz' saved [443844/443844]
Building target platforms: i686-linux
Wrote: /home/users/builder/rpm/SRPMS/autoconf2_13-2.13-1.src.rpm
exit status 0
Tagging with prefix: auto/ti/
HEAD is now at 2a55bfc... - verbose files, age note (it's almost 18y old...) - adjusted info patch
fatal: HEAD does not point to a branch
Writing git revision for tag auto/ti/autoconf2_13-2.13-1
On detached HEAD. Use -r BRANCHNAME to override
error: Macro %_builddir has empty body
error: Macro %_builddir has empty body
Executing(--clean): env -i TMP=/home/users/builder/tmp HOME=/home/users/builder PATH=/bin:/usr/bin:/usr/sbin:/sbin:/usr/X11R6/bin TMPDIR=/home/users/builder/tmp  /bin/sh -e /home/users/builder/tmp/rpm-tmp.42328
+ umask 022
+ cd /home/users/builder/rpm/BUILD
+ rm -rf autoconf-2.13
Begin-TLD-Builder-Info
Build-Time: user:0.74s sys:0.26s real:2.57s (faults io:0 non-io:156407)
Files queued for ftp:
    470171 autoconf2_13-2.13-1.src.rpm
       189 autoconf2_13-2.13-1.src.rpm.uploadinfo
End-TLD-Builder-Info