internal/vcs: show stderr when git exits with an error

For example, rather than the unhelpful error

    running "git" ["-c" "log.showsignature=false" "log" "-1" "--format=%H:%ct"]: exit status 128

we would now show

    running "git" ["-c" "log.showsignature=false" "log" "-1" "--format=%H:%ct"]: exit status 128: fatal: your current branch 'master' does not have any commits yet

which is arguably quite long, but at least it's useful to understand
why git is failing.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Ia7e40cb59fce41b3ec096bb95977cdeb11cea01b
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1198173
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Matthew Sackman <matthew@cue.works>

internal/vcs/vcs.go

@@ -17,6 +17,7 @@
 package vcs
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"os"
@@ -115,7 +116,10 @@ func runCmd(ctx context.Context, dir string, cmdName string, args ...string) (st
 	cmd.Dir = dir
 
 	out, err := cmd.Output()
-	if err != nil {
+	if exitErr, ok := err.(*exec.ExitError); ok {
+		// git's stderr often ends with a newline, which is unnecessary.
+		return "", fmt.Errorf("running %q %q: %v: %s", cmdName, args, err, bytes.TrimSpace(exitErr.Stderr))
+	} else if err != nil {
 		return "", fmt.Errorf("running %q %q: %v", cmdName, args, err)
 	}
 	return string(out), nil