[...]

do so (now or later) by using -b with the checkout command again. Example:
  git checkout -b new_branch_name
HEAD is now at b018d5f... - updated to 2.1.21 - removed obsolete build,xserver-1.19 patches
fatal: HEAD does not point to a branch
Available branches: master
Searching for tag auto/ti/xorg-driver-video-nv-2.1.21-1...
Searching for tag auto/ti/xorg-driver-video-nv-2_1_21-1...
--2018-03-15 09:18:03--  http://distfiles.pld-linux.org/distfiles/by-md5/1/f/1fdd41d88232043811cbe3f5ac724c5f/xf86-video-nv-2.1.21.tar.bz2
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: 424091 (414K) [application/x-bzip2]
Saving to: 'xf86-video-nv-2.1.21.tar.bz2'
     0K .......... .......... .......... .......... .......... 12% 1.21M 0s
    50K .......... .......... .......... .......... .......... 24% 2.40M 0s
   100K .......... .......... .......... .......... .......... 36% 87.4M 0s
   150K .......... .......... .......... .......... .......... 48% 74.7M 0s
   200K .......... .......... .......... .......... .......... 60% 2.57M 0s
   250K .......... .......... .......... .......... .......... 72% 75.8M 0s
   300K .......... .......... .......... .......... .......... 84% 84.4M 0s
   350K .......... .......... .......... .......... .......... 96% 71.0M 0s
   400K .......... ....                                       100%  489M=0.08s
2018-03-15 09:18:03 (4.89 MB/s) - 'xf86-video-nv-2.1.21.tar.bz2' saved [424091/424091]
Building target platforms: i686-linux
Wrote: /home/users/builder/rpm/SRPMS/xorg-driver-video-nv-2.1.21-1.src.rpm
exit status 0
Tagging with prefix: auto/ti/
HEAD is now at b018d5f... - updated to 2.1.21 - removed obsolete build,xserver-1.19 patches
fatal: HEAD does not point to a branch
Writing git revision for tag auto/ti/xorg-driver-video-nv-2.1.21-1
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.33841
+ umask 022
+ cd /home/users/builder/rpm/BUILD
+ rm -rf xf86-video-nv-2.1.21
Begin-TLD-Builder-Info
Build-Time: user:0.52s sys:0.11s real:2.42s (faults io:0 non-io:91627)
Files queued for ftp:
    431388 xorg-driver-video-nv-2.1.21-1.src.rpm
       199 xorg-driver-video-nv-2.1.21-1.src.rpm.uploadinfo
End-TLD-Builder-Info