ci: add a problem matcher for GitHub Actions

With this patch, test failures will be annotated with a helpful,
clickable message in GitHub Actions. For details, see
https://github.com/actions/toolkit/blob/master/docs/problem-matchers.md

Note: we need to set `TEST_SHELL_PATH` to Bash so that the problem
matcher is fed a file and line number for each test failure.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/git-problem-matcher.json

@@ -0,0 +1,16 @@
+{
+    "problemMatcher": [
+        {
+            "owner": "git-test-suite",
+            "pattern": [
+                {
+                    "regexp": "^([^ :]+\\.sh):(\\d+): (error|warning|info):\\s+(.*)$",
+                    "file": 1,
+                    "line": 2,
+                    "severity": 3,
+                    "message": 4
+                }
+            ]
+        }
+    ]
+}

ci/lib.sh

@@ -157,6 +157,11 @@ then
 	MAKEFLAGS="$MAKEFLAGS --jobs=10"
 	test windows != "$CI_OS_NAME" ||
 	GIT_TEST_OPTS="--no-chain-lint --no-bin-wrappers $GIT_TEST_OPTS"
+
+	# https://github.com/actions/toolkit/blob/master/docs/commands.md#problem-matchers
+	echo "::add-matcher::ci/git-problem-matcher.json"
+	test linux-musl = "$jobname" ||
+	MAKEFLAGS="$MAKEFLAGS TEST_SHELL_PATH=/bin/sh"
 else
 	echo "Could not identify CI type" >&2
 	env >&2