.tools: Run 'git show --check ...' on each commit

Watch for accidentally-injected trailing whitespace.  The stdout for
failing checks is a bit hard to read:

  not ok 2 - git show --check 2652b57a368e8fdd6745f2354f25c56e57326b16
   ---
   message: "commit 2652b57a368e8fdd6745f2354f25c56e57326b16\nAuthor: W. Trevor King
     <wking@tremily.us>\nDate:   Sat Sep 12 08:32:15 2015 -0700\n\n    .tools: Run 'git
     show --check ...' on each commit\n    \n    Watch for accidentally-injected trailing
     whitespace.  The stdout for\n    failing checks is a bit hard to read:\n    \n      not
     ok 2 - exit status 2\n       ---\n       message: \"commit 09748ebfd739dbcee35a585a75f896a74aa1c2f0\\nAuthor:
     W. Trevor King\n         <wking@tremily.us>\\nDate:   Sat Sep 12 08:32:15 2015 -0700\\n\\n
     \   .tools: Run 'git\n         show --check ...' on each commit\\n    \\n    Watch
     for accidentally-injected trailing\n         whitespace.\\n    \\n    Signed-off-by:
     W. Trevor King <wking@tremily.us>\\n\\n.tools/validate.go:56:\n         trailing
     whitespace.\\n+\\t\\t\\treturn vr  \\n\"\n       ...\n    \n    Signed-off-by: W.
     Trevor King <wking@tremily.us>\n\n.tools/validate.go:57: trailing whitespace.\n+\t\t\treturn
     vr \n"
   ...

But having the command in the "not ok" line should make it easy for
folks to reproduce locally if they're not collecting the YAML blocks
with a TAP harness.

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking

.tools/validate.go

@@ -43,6 +43,22 @@ var DefaultRules = []ValidateRule{
 		return vr
 	},
 	// TODO add something for the cleanliness of the c.Subject
+	func(c CommitEntry) (vr ValidateResult) {
+		vr.CommitEntry = c
+		buf := bytes.NewBuffer([]byte{})
+		args := []string{"git", "show", "--check", c["commit"]}
+		vr.Msg = strings.Join(args, " ")
+		cmd := exec.Command(args[0], args[1:]...)
+		cmd.Stdout = buf
+		cmd.Stderr = os.Stderr
+		if err := cmd.Run(); err != nil {
+			vr.Pass = false
+			vr.Detail = string(buf.Bytes())
+			return vr
+		}
+		vr.Pass = true
+		return vr
+	},
 	func(c CommitEntry) (vr ValidateResult) {
 		return ExecTree(c, "go", "vet", "./...")
 	},