[...]

do so (now or later) by using -b with the checkout command again. Example:
  git checkout -b new_branch_name
HEAD is now at 87b72c9... - updated gettext BR
fatal: HEAD does not point to a branch
Available branches: AC-branch master
--2015-07-28 10:32:54--  http://distfiles.pld-linux.org/distfiles/by-md5/9/7/97639f8821b10f80943fa17da302607e/mcrypt-2.6.8.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: 471915 (461K) [application/x-gzip]
Saving to: 'mcrypt-2.6.8.tar.gz'
     0K .......... .......... .......... .......... .......... 10%  779K 1s
    50K .......... .......... .......... .......... .......... 21% 1.47M 0s
   100K .......... .......... .......... .......... .......... 32% 1.53M 0s
   150K .......... .......... .......... .......... .......... 43% 1.60M 0s
   200K .......... .......... .......... .......... .......... 54% 10.8M 0s
   250K .......... .......... .......... .......... .......... 65% 1.72M 0s
   300K .......... .......... .......... .......... .......... 75% 11.0M 0s
   350K .......... .......... .......... .......... .......... 86% 10.9M 0s
   400K .......... .......... .......... .......... .......... 97% 1.90M 0s
   450K ..........                                            100% 10.7M=0.2s
2015-07-28 10:32:54 (1.97 MB/s) - 'mcrypt-2.6.8.tar.gz' saved [471915/471915]
warning: Could not canonicalize hostname: giant
Building target platforms: i686-linux
Wrote: /home/users/builder/rpm/SRPMS/mcrypt-2.6.8-1.src.rpm
exit status 0
Tagging with prefix: auto/ti/
HEAD is now at 87b72c9... - updated gettext BR
fatal: HEAD does not point to a branch
Writing git revision for tag auto/ti/mcrypt-2.6.8-1
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.78835
+ umask 022
+ cd /home/users/builder/rpm/BUILD
+ rm -rf mcrypt-2.6.8
Begin-PLD-Builder-Info
Build-Time: user:2.84s sys:1.42s real:5.17s (faults io:0 non-io:226751)
Files queued for ftp:
    486932 mcrypt-2.6.8-1.src.rpm
       184 mcrypt-2.6.8-1.src.rpm.uploadinfo
End-PLD-Builder-Info