Return an error if Version() prints to stderr

We call `hg --debug identify` to get the version, but this command can
print warnings to stderr. Previously we would read the first line as
the version, which would try to parse whatever warning was printed as
the version.

Instead parse stdout and stderr separately and return an error if
anything gets sent to stderr. This should at least alert people to the
problem and avoid percolating problems further downstream.

Fixes #90.

hg.go

@@ -1,7 +1,9 @@
 package vcs
 
 import (
+	"bytes"
 	"encoding/xml"
+	"errors"
 	"os"
 	"os/exec"
 	"regexp"
@@ -110,12 +112,20 @@ func (s *HgRepo) UpdateVersion(version string) error {
 
 // Version retrieves the current version.
 func (s *HgRepo) Version() (string, error) {
-	out, err := s.RunFromDir("hg", "--debug", "identify")
-	if err != nil {
-		return "", NewLocalError("Unable to retrieve checked out version", err, string(out))
-	}
-
-	parts := strings.SplitN(string(out), " ", 2)
+	c := s.CmdFromDir("hg", "--debug", "identify")
+	stdout, stderr := new(bytes.Buffer), new(bytes.Buffer)
+	c.Stdout = stdout
+	c.Stderr = stderr
+	if err := c.Run(); err != nil {
+		return "", NewLocalError("Unable to retrieve checked out version", err, stderr.String())
+	}
+	if stderr.Len() > 0 {
+		// "hg --debug identify" can print out errors before it actually prints
+		// the version.
+		// https://github.com/Masterminds/vcs/issues/90
+		return "", NewLocalError("Unable to retrieve checked out version", errors.New("Error output printed before identify"), stderr.String())
+	}
+	parts := strings.SplitN(stdout.String(), " ", 2)
 	sha := parts[0]
 	return strings.TrimSpace(sha), nil
 }

hg_test.go

@@ -2,12 +2,11 @@ package vcs
 
 import (
 	"io/ioutil"
+	"os"
 	"path/filepath"
 	"strings"
-	"time"
-	//"log"
-	"os"
 	"testing"
+	"time"
 )
 
 // Canary test to ensure HgRepo implements the Repo interface.