[MINOR][BUILD] Decode output of commands during merge script as UTF-8 consistently
[MINOR][BUILD] Decode output of commands during merge script as UTF-8 consistently

### What changes were proposed in this pull request?

In the PR merge script, decode the raw output of subprocess commands like `git` using UTF-8 encoding, consistently.

### Why are the changes needed?

The merge script occasionally fails if run with Python 2 and the output of a command like `git` contains non-ASCII characters. I think this most usually happens when a user name, for example, contains Chinese characters.

This is because the output is decoded according to `sys.getdefaultencoding()`, which is ASCII in Python 2. It's UTF-8 in Python 3, by default.

### Does this PR introduce any user-facing change?

No.

### How was this patch tested?

The change caused a merge that failed before to succeed.

Closes #25991 from srowen/MergePRUTF8.

Authored-by: Sean Owen <3f9d48d7c3f24f7009479c5fa630bd1131ffc724@databricks.com>
Signed-off-by: HyukjinKwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

dbtsai/spark,maropu/spark,JoshRosen/spark,BryanCutler/spark,dongjoon-hyun/spark,jiangxb1987/spark,dbtsai/spark,milliman/spark,darionyaphet/spark,ptkool/spark,cloud-fan/spark,srowen/spark,witgo/spark,skonto/spark,jkbradley/spark,nchammas/spark,zuotingbing/spark,wzhfy/spark,zero323/spark,zzcclp/spark,skonto/spark,dongjoon-hyun/spark,wangmiao1981/spark,JoshRosen/spark,srowen/spark,rezasafi/spark,wangyum/spark,wangyum/spark,shuangshuangwang/spark,JoshRosen/spark,maropu/spark,wangyum/spark,wangmiao1981/spark,milliman/spark,darionyaphet/spark,wzhfy/spark,ptkool/spark,ueshin/apache-spark,BryanCutler/spark,JoshRosen/spark,rednaxelafx/apache-spark,holdenk/spark,xuanyuanking/spark,dongjoon-hyun/spark,shuangshuangwang/spark,dbtsai/spark,caneGuy/spark,mahak/spark,WeichenXu123/spark,spark-test/spark,chuckchen/spark,cloud-fan/spark,JoshRosen/spark,nchammas/spark,rezasafi/spark,chuckchen/spark,nchammas/spark,milliman/spark,nchammas/spark,rednaxelafx/apache-spark,xuanyuanking/spark,apache/spark,maropu/spark,skonto/spark,chuckchen/spark,goldmedal/spark,shaneknapp/spark,milliman/spark,darionyaphet/spark,vinodkc/spark,spark-test/spark,jiangxb1987/spark,ptkool/spark,srowen/spark,goldmedal/spark,cloud-fan/spark,WeichenXu123/spark,caneGuy/spark,gengliangwang/spark,wangyum/spark,taroplus/spark,kevinyu98/spark,shuangshuangwang/spark,goldmedal/spark,spark-test/spark,dbtsai/spark,zero323/spark,HyukjinKwon/spark,WeichenXu123/spark,rednaxelafx/apache-spark,milliman/spark,srowen/spark,taroplus/spark,jkbradley/spark,vinodkc/spark,apache/spark,skonto/spark,kevinyu98/spark,ptkool/spark,nchammas/spark,apache/spark,ConeyLiu/spark,ptkool/spark,dbtsai/spark,wzhfy/spark,xuanyuanking/spark,wangyum/spark,BryanCutler/spark,rezasafi/spark,wzhfy/spark,goldmedal/spark,cloud-fan/spark,WeichenXu123/spark,mahak/spark,gengliangwang/spark,taroplus/spark,zero323/spark,zzcclp/spark,taroplus/spark,witgo/spark,vinodkc/spark,caneGuy/spark,hvanhovell/spark,ueshin/apache-spark,taroplus/spark,dongjoon-hyun/spark,caneGuy/spark,xuanyuanking/spark,milliman/spark,jkbradley/spark,hvanhovell/spark,xuanyuanking/spark,darionyaphet/spark,wangmiao1981/spark,skonto/spark,zzcclp/spark,jiangxb1987/spark,wangmiao1981/spark,milliman/spark,HyukjinKwon/spark,zero323/spark,jiangxb1987/spark,rednaxelafx/apache-spark,taroplus/spark,zero323/spark,jkbradley/spark,spark-test/spark,darionyaphet/spark,vinodkc/spark,holdenk/spark,zuotingbing/spark,rednaxelafx/apache-spark,rednaxelafx/apache-spark,holdenk/spark,chuckchen/spark,zuotingbing/spark,jiangxb1987/spark,holdenk/spark,vinodkc/spark,zero323/spark,wangmiao1981/spark,rezasafi/spark,shuangshuangwang/spark,ueshin/apache-spark,zzcclp/spark,zuotingbing/spark,matthewfranglen/spark,ueshin/apache-spark,nchammas/spark,darionyaphet/spark,hvanhovell/spark,HyukjinKwon/spark,HyukjinKwon/spark,goldmedal/spark,taroplus/spark,zuotingbing/spark,ConeyLiu/spark,kevinyu98/spark,JoshRosen/spark,spark-test/spark,kevinyu98/spark,wangmiao1981/spark,wzhfy/spark,shaneknapp/spark,srowen/spark,maropu/spark,rezasafi/spark,zuotingbing/spark,dongjoon-hyun/spark,BryanCutler/spark,zero323/spark,shaneknapp/spark,gengliangwang/spark,witgo/spark,apache/spark,shaneknapp/spark,shaneknapp/spark,wangyum/spark,dbtsai/spark,ConeyLiu/spark,mahak/spark,BryanCutler/spark,nchammas/spark,jiangxb1987/spark,witgo/spark,apache/spark,rezasafi/spark,gengliangwang/spark,hvanhovell/spark,jiangxb1987/spark,holdenk/spark,holdenk/spark,mahak/spark,BryanCutler/spark,xuanyuanking/spark,goldmedal/spark,mahak/spark,hvanhovell/spark,ptkool/spark,vinodkc/spark,wangyum/spark,gengliangwang/spark,spark-test/spark,hvanhovell/spark,srowen/spark,ConeyLiu/spark,HyukjinKwon/spark,skonto/spark,chuckchen/spark,caneGuy/spark,dbtsai/spark,HyukjinKwon/spark,wangmiao1981/spark,jkbradley/spark,WeichenXu123/spark,ConeyLiu/spark,rednaxelafx/apache-spark,maropu/spark,ConeyLiu/spark,witgo/spark,skonto/spark,chuckchen/spark,ptkool/spark,ueshin/apache-spark,kevinyu98/spark,wzhfy/spark,srowen/spark,shaneknapp/spark,caneGuy/spark,mahak/spark,cloud-fan/spark,kevinyu98/spark,caneGuy/spark,gengliangwang/spark,ConeyLiu/spark,apache/spark,HyukjinKwon/spark,wzhfy/spark,BryanCutler/spark,shuangshuangwang/spark,jkbradley/spark,JoshRosen/spark,goldmedal/spark,shaneknapp/spark,apache/spark,xuanyuanking/spark,zuotingbing/spark,zzcclp/spark,dongjoon-hyun/spark,witgo/spark,ueshin/apache-spark,spark-test/spark,gengliangwang/spark,cloud-fan/spark,shuangshuangwang/spark,holdenk/spark,rezasafi/spark,witgo/spark,hvanhovell/spark,vinodkc/spark,shuangshuangwang/spark,zzcclp/spark,WeichenXu123/spark,maropu/spark,zzcclp/spark,cloud-fan/spark,maropu/spark,jkbradley/spark,ueshin/apache-spark,chuckchen/spark,WeichenXu123/spark,darionyaphet/spark,mahak/spark,dongjoon-hyun/spark,kevinyu98/spark