[AIRFLOW-6818] Fix for old versions of git on Dockerhub builds (#7440)
[AIRFLOW-6818] Fix for old versions of git on Dockerhub builds (#7440)

Dockerhub uses an old version of git (2.7.4 as of our testing in Feb
2020) so needs a slightly different syntax for git ls-files with an
exclusion. This works with current-latest git (2.25.0) too
mistercrunch/airflow,lyft/incubator-airflow,mrkm4ntr/incubator-airflow,wooga/airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,DinoCow/airflow,bolkedebruin/airflow,sekikn/incubator-airflow,apache/incubator-airflow,lyft/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,apache/airflow,dhuang/incubator-airflow,dhuang/incubator-airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,airbnb/airflow,wooga/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,apache/airflow,sekikn/incubator-airflow,mrkm4ntr/incubator-airflow,mtagle/airflow,cfei18/incubator-airflow,wooga/airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,apache/airflow,nathanielvarona/airflow,mtagle/airflow,airbnb/airflow,DinoCow/airflow,mtagle/airflow,nathanielvarona/airflow,sekikn/incubator-airflow,danielvdende/incubator-airflow,airbnb/airflow,DinoCow/airflow,airbnb/airflow,apache/incubator-airflow,wooga/airflow,apache/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,mistercrunch/airflow,apache/airflow,bolkedebruin/airflow,dhuang/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,DinoCow/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,mrkm4ntr/incubator-airflow,sekikn/incubator-airflow,nathanielvarona/airflow,mtagle/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,danielvdende/incubator-airflow