Add optional JACOCO_SOURCE_PATH env variable

Add an optional `JACOCO_SOURCE_PATH` environment variable to fix
bug where sometimes the path to source code can't be inferred.

Example usage:

    JACOCO_SOURCE_PATH=src/main/java ./cc-test-reporter \
      format-coverage target/site/jacoco/jacoco.xml     \
      --input-type jacoco

Addresses #259.

formatters/jacoco/jacoco.go

@@ -4,16 +4,22 @@ import (
 	"encoding/xml"
 	"fmt"
 	"os"
+	"path"
 	"strings"
 
 	"github.com/Sirupsen/logrus"
 	"github.com/codeclimate/test-reporter/env"
 	"github.com/codeclimate/test-reporter/formatters"
+	"github.com/gobuffalo/envy"
 	"github.com/pkg/errors"
 )
 
 var searchPaths = []string{"jacoco.xml"}
 
+func getSourcePath() string {
+	return envy.Get("JACOCO_SOURCE_PATH", "")
+}
+
 type Formatter struct {
 	Path string
 }
@@ -32,6 +38,8 @@ func (f *Formatter) Search(paths ...string) (string, error) {
 }
 
 func (r Formatter) Format() (formatters.Report, error) {
+	sourcePath := getSourcePath()
+
 	rep, err := formatters.NewReport()
 	if err != nil {
 		return rep, err
@@ -53,7 +61,7 @@ func (r Formatter) Format() (formatters.Report, error) {
 		for _, xmlSF := range xmlPackage.SourceFile {
 			num := 1
 			filepath := fmt.Sprintf("%s/%s", xmlPackage.Name, xmlSF.Name)
-			sf, err := formatters.NewSourceFile(filepath, gitHead)
+			sf, err := formatters.NewSourceFile(path.Join(sourcePath, filepath), gitHead)
 			if err != nil {
 				return rep, errors.WithStack(err)
 			}

formatters/jacoco/jacoco_test.go

@@ -6,6 +6,8 @@ import (
 	"gopkg.in/src-d/go-git.v4/plumbing/object"
 
 	"github.com/codeclimate/test-reporter/env"
+	"github.com/codeclimate/test-reporter/formatters"
+	"github.com/gobuffalo/envy"
 	"github.com/stretchr/testify/require"
 )
 
@@ -31,3 +33,31 @@ func Test_Parse(t *testing.T) {
 	r.Equal(3, sf.Coverage[6].Int)
 	r.Equal(0, sf.Coverage[8].Int)
 }
+
+func Test_Parse_SourcePath(t *testing.T) {
+	gb := env.GitBlob
+	defer func() { env.GitBlob = gb }()
+	env.GitBlob = func(s string, c *object.Commit) (string, error) {
+		return s, nil
+	}
+
+	r := require.New(t)
+
+	f := &Formatter{Path: "./example.xml"}
+	var rep formatters.Report
+	var err error
+	envy.Temp(func() {
+		envy.Set("JACOCO_SOURCE_PATH", "src/main/java")
+		rep, err = f.Format()
+	})
+	r.NoError(err)
+	r.Len(rep.SourceFiles, 3)
+
+	sf := rep.SourceFiles["src/main/java/be/apo/basic/Application.java"]
+	r.InDelta(33.3, sf.CoveredPercent, 1)
+	r.Len(sf.Coverage, 11)
+	r.True(sf.Coverage[6].Valid)
+	r.False(sf.Coverage[8].Valid)
+	r.Equal(3, sf.Coverage[6].Int)
+	r.Equal(0, sf.Coverage[8].Int)
+}