Check scripts with shellcheck before accepting new commands

Only check scripts that have been added in the current pull request.

Do not check them on macOS as homebrew is slow to install shellcheck and
the macOS cut command does not support an empty -d argument.

.travis.yml

@@ -5,8 +5,10 @@ addons:
   apt:
     packages:
     - bsdmainutils
+    - shellcheck
 script:
   - ./check_integrity.sh $(find bin | cut -b 5- | xargs)
+  - ./check_changed_files.sh
   # Test for the brew release
   - mkdir ../release && git archive --format=tar.gz HEAD > ../release/git-extras-release.tar.gz
   - cd ../release

check_changed_files.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+
+err() {
+    echo >&2 "$1"
+    exit 1
+}
+
+if command -v shellcheck > /dev/null ; then
+  # Check all changed shell scripts
+  if ! git diff -z --name-only "$TRAVIS_BRANCH...HEAD" | xargs -0 ./shellcheck.sh ; then
+    err "Please fix the shellcheck issues"
+  fi
+fi

shellcheck.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+files=()
+for f ; do
+  if [ -r "$f" ] ; then
+    file --mime-type -r0 "$f" |
+    cut -d "" -f 2 |
+    grep -q "^: text/x-shellscript$" &&
+    files+=("$f")
+  fi
+done
+if [ ${#files[@]} -gt 0 ] ; then
+  shellcheck "${files[@]}"
+  ret=$?
+  if [ $ret -eq 1 ] || [ $ret -eq 2 ] ; then
+    exit 1
+  fi
+fi