Add markdownlint in pre-commit (sql-machine-learning#2028)

* Add markdownlint in pre-commit

* List markdown files in pre-commit.sh

* List all the markdown files to run the markdownlint check

* Temp update on cli.md

* Try pre-commit with from-ref and to-ref

* Trial: run pre-commit only on changed files

* Print CHANGED_FILES for debugging.

* DEBUG CI: Check git branch on Travis VM

* Revert pre-commit.sh to the initial version

* Run pre-commit for the difference between develop branch and the current PR branch.

* Remove the git command for printing branch

.pre-commit-config.yaml

@@ -60,3 +60,11 @@ repos:
     rev: v1.23.0
     hooks:
       - id: yamllint
+  - repo: https://github.com/igorshubovych/markdownlint-cli
+    rev: v0.23.1
+    hooks:
+      - id: markdownlint
+        name: markdownlint
+        description: "Checks the style of Markdown files."
+        entry: markdownlint
+        language: node

docs/designs/cli.md

@@ -26,7 +26,7 @@ discusssing about this command-line client tool.
     requirements.txt
     ```
 
-2. Generate a Dockerfile.  
+1. Generate a Dockerfile.  
 
     Input the command:
 
@@ -47,19 +47,20 @@ discusssing about this command-line client tool.
 
     Users can make additional updates on the Dockerfile if necessary.
 
-3. Build the Docker image for an ElasticDL job.
+1. Build the Docker image for an ElasticDL job.
 
     ```bash
-    elasticdl zoo build --image=reg.docker.alibaba-inc.com/bright/elasticdl-wnd:1.0 .
+    elasticdl zoo build
+        --image=reg.docker.alibaba-inc.com/bright/elasticdl-wnd:1.0 .
     ```
 
-4. Push the Docker image to a remote registry (optional)
+1. Push the Docker image to a remote registry (optional)
 
     ```bash
     elasticdl zoo push reg.docker.alibaba-inc.com/bright/elasticdl-wnd:1.0
     ```
 
-5. Submit a model training/prediction/evaluation job.
+1. Submit a model training/prediction/evaluation job.
 
     ```bash
     elasticdl train \

scripts/pre-commit.sh

@@ -14,14 +14,6 @@
 
 
 
-PYTHON_SOURCE_FILES=$(find elasticdl/python \
-                           elasticdl_preprocessing \
-                           model_zoo \
-                           tools \
-                           setup.py \
-                           scripts \
-                           -name '*.py' -print0 | tr '\0' ' ')
-
-GO_SOURCE_FILES=$(find elasticdl/go/ -name '*.go' -print0 | tr '\0' ' ')
-
-pre-commit run --files "$PYTHON_SOURCE_FILES $GO_SOURCE_FILES"
+PROJECT_URL=https://github.com/sql-machine-learning/elasticdl.git
+REMOTE=$(git remote -v | grep $PROJECT_URL | cut -f1 | head -1)
+pre-commit run --from-ref "$REMOTE/develop" --to-ref HEAD