kernel: Fix pull when we are not currently on a branch (tiann#388)

Before fix:
```
[celica@arch msm-5.4]$ curl -LSs "https://raw.githubusercontent.com/tiann/KernelSU/main/kernel/setup.sh" | bash -
++ pwd
+ GKI_ROOT=/home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4
+ echo '[+] GKI_ROOT: /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4'
[+] GKI_ROOT: /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4
+ test -d /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/common/drivers
+ test -d /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/drivers
+ DRIVER_DIR=/home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/drivers
+ test -d /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/KernelSU
+ cd /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/KernelSU
+ git stash
没有要保存的本地修改
+ git pull
您当前不在一个分支上。
请指定您要合并哪一个分支。
详见 git-pull(1)。

    git pull <远程> <分支>
```

After fix:
```
[celica@arch msm-5.4]$ curl -LSs "https://raw.githubusercontent.com/natsumerinchan/KernelSU/main/kernel/setup.sh" | bash -s main
++ pwd
+ GKI_ROOT=/home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4
+ echo '[+] GKI_ROOT: /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4'
[+] GKI_ROOT: /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4
+ test -d /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/common/drivers
+ test -d /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/drivers
+ DRIVER_DIR=/home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/drivers
+ test -d /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/KernelSU
+ cd /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/KernelSU
+ git stash
没有要保存的本地修改
++ git status
++ grep -Po 'v\d+(\.\d+)*'
++ head -n1
+ '[' v0.5.2 ']'
+ git checkout main
之前的 HEAD 位置是 0bda101 ci: fix WSA upload path (tiann#372)
切换到分支 'main'
您的分支与上游分支 'origin/main' 一致。
+ git pull
已经是最新的。
+ '[' -z main ']'
+ git checkout main
已经位于 'main'
您的分支与上游分支 'origin/main' 一致。
+ cd /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4
+ echo '[+] GKI_ROOT: /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4'
[+] GKI_ROOT: /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4
+ echo '[+] Copy kernel su driver to /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/drivers'
[+] Copy kernel su driver to /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/drivers
+ test -e /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/drivers/kernelsu
+ echo '[+] Add kernel su driver to Makefile'
[+] Add kernel su driver to Makefile
+ DRIVER_MAKEFILE=/home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/drivers/Makefile
+ grep -q kernelsu /home/celica/workdir/mvaisakh-kernel/kernel/msm-5.4/drivers/Makefile
+ echo '[+] Done.'
[+] Done.
```

kernel/setup.sh

@@ -17,7 +17,11 @@ fi
 
 test -d "$GKI_ROOT/KernelSU" || git clone https://github.com/tiann/KernelSU
 cd "$GKI_ROOT/KernelSU"
-git stash && git pull
+git stash
+if [ "$(git status | grep -Po 'v\d+(\.\d+)*' | head -n1)" ]; then
+     git checkout main
+fi
+git pull
 if [ -z "${1-}" ]; then
     git checkout "$(git describe --abbrev=0 --tags)"
 else