Switch from Travis to Github Actions CI (#2041)

* Fix .run_formal_checks for Github Actions instead of Travis
* Remove .run_chisel_tests

Because we publish SNAPSHOTs on every merge to master, the Chisel repo
checks against master of FIRRTL regularly

* Only run unidoc for Scala 2.12

Under Travis we ran for 2.11, 2.12, and 2.13, but it doesn't work when
using ++2.<major>.<minor> when major != to 2.12. It seems if we want to
run all 3, we have to run as `sbt +unidoc`. It's not clear how much
benefit we get from building on the other versions, so stick with 2.12
for now.

* Generate .mergify.yml with script in chisel-repo-tools

Generated with config:

conditions:
  - status-success=all tests passed
branches:
  - 1.2.x
  - 1.3.x
  - 1.4.x

1 files changed, 14 insertions, 12 deletions

diff --git a/.run_formal_checks.sh b/.run_formal_checks.sh
index f5ffec25..0a28a1d1 100755
--- a/.run_formal_checks.sh
+++ b/.run_formal_checks.sh
@@ -11,23 +11,25 @@ fi
 
 DUT=$1
 
-# Run formal check only for PRs
-if [ $TRAVIS_PULL_REQUEST = "false" ]; then
-    echo "Not a pull request, no formal check"
-    exit 0
-else
-    # $TRAVIS_BRANCH is branch targeted by PR
-    # Travis does a shallow clone, checkout PR target so that we have it
-    # THen return to previous branch so HEAD points to the commit we're testing
-    git remote set-branches origin $TRAVIS_BRANCH && git fetch
-    git checkout $TRAVIS_BRANCH
+# See https://docs.github.com/en/actions/reference/environment-variables
+# for info about these variables
+
+# Run formal check only for PRs, GITHUB_BASE_REF is only set for PRs
+if [ ! -z "$GITHUB_BASE_REF" ]; then
+    # Github Actions does a shallow clone, checkout PR target so that we have it
+    # Then return to previous branch so HEAD points to the commit we're testing
+    git remote set-branches origin $GITHUB_BASE_REF && git fetch
+    git checkout $GITHUB_BASE_REF
     git checkout -
     # Skip if '[skip formal checks]' shows up in any of the commit messages in the PR
-    if git log --format=%B --no-merges $TRAVIS_BRANCH..HEAD | grep '\[skip formal checks\]'; then
+    if git log --format=%B --no-merges $GITHUB_BASE_REF..HEAD | grep '\[skip formal checks\]'; then
         echo "Commit message says to skip formal checks"
         exit 0
     else
         cp regress/$DUT.fir $DUT.fir
-        ./scripts/formal_equiv.sh HEAD $TRAVIS_BRANCH $DUT
+        ./scripts/formal_equiv.sh HEAD $GITHUB_BASE_REF $DUT
     fi
+else
+    echo "Not a pull request, no formal check"
+    exit 0
 fi