Add CI test to ensure that README.md files are present and correct

Checking readmes takes up non-trivial maintainer time, and is
prone to error. This script adds a check to the CI which ensures
that PRs are only accepted if their README.md files are
present and correct.

It begins by isolating the exercises already impacted by the PR,
so that upstream changes to unrelated exercises don't cause undue
difficulty. It then simply ensures that the README.md within
the PR is identical to the one generated by `configlet generate`.

Outputs lists of exercises with missing or incorrect READMEs,
so people for whom CI fails due to this test not passing should
have an easy time understanding what went wrong and how to fix it.

.travis.yml

@@ -6,6 +6,7 @@ script:
 - "sh ./_test/ensure-stubs-compile.sh"
 - "sh ./_test/count-ignores.sh"
 - "./bin/fetch-configlet"
+- "sh ./_test/ensure-readmes-are-updated.sh"
 - "./bin/configlet lint ."
 sudo: false
 rust:

_test/ensure-readmes-are-updated.sh

@@ -0,0 +1,36 @@
+if [ ! -x bin/configlet ]; then
+   echo "Improper configuration; configlet should exist in bin/ when this script is run"
+   echo "Ping a Rust track maintainer to fix this"
+   exit 1
+fi
+
+newline=$'\n  '
+
+missing_readmes=""
+wrong_readmes=""
+for exercise in $(git diff --name-only master..$(git rev-parse --abbrev-ref HEAD) | grep exercises/ | cut -d'/' -f2 -s | sort -fu); do
+   echo "Checking readme for $exercise"
+   readme_path="exercises/${exercise}/README.md"
+   if [ ! -f $readme_path ]; then
+      missing_readmes="$missing_readmes$newline$exercise"
+   else
+      existing_readme_checksum=$(md5sum $readme_path | cut -d' ' -f1)
+      # generate the new README
+      bin/configlet generate . --only "$exercise"
+      generated_readme_checksum=$(md5sum $readme_path | cut -d' ' -f1)
+
+      if [ $existing_readme_checksum != $generated_readme_checksum ]; then
+         wrong_readmes="$wrong_readmes$newline$exercise"
+      fi
+   fi
+done
+
+if [ -n "$missing_readmes" ]; then
+  echo "Exercises missing README.md:$missing_readmes"
+fi
+if [ -n "$wrong_readmes" ]; then
+  echo "Exercises with out-of-date README.md:$wrong_readmes"
+fi
+if [ -n "$missing_readmes" -o -n "$wrong_readmes" ]; then
+   exit 1
+fi