Update default values to match 50/72 rule

The 50/72 rule was laid out by tpope in 2008 (see
www.tpope.net/node/106), and has gained wide acceptance
in the git community. It makes sense to enforce it by
default, since the values are configurable anyway.

Change-Id: I472ddbd2caf7cd1036f8ce2953380445bca8712a
diff --git a/src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java b/src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java
index 18f83eb..0a7070f 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java
@@ -30,8 +30,8 @@
 public class CommitMessageLengthValidation implements CommitValidationListener {
-  private static final int DEFAULT_MAX_SUBJECT_LENGTH = 65;
-  private static final int DEFAULT_MAX_LINE_LENGTH = 70;
+  private static final int DEFAULT_MAX_SUBJECT_LENGTH = 50;
+  private static final int DEFAULT_MAX_LINE_LENGTH = 72;
   private static final int DEFAULT_LONG_LINES_THRESHOLD = 33;
   private static final boolean DEFAULT_REJECT_TOO_LONG = false;
   private static final String COMMIT_MESSAGE_SECTION = "commitmessage";
diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index b33ce93..51e7cd4 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -3,14 +3,18 @@
 The maximum lengths of the subject and message body can be
 configured in the standard Gerrit config file `gerrit.config`.
+The defaults are 50 characters for the summary and 72 characters
+for the description, as recommended by the
+[git tutorial](https://kernel.googlesource.com/pub/scm/git/git/+/927a503cd07718ea0f700052043f383253904a56/Documentation/tutorial.txt#64)
+and [expanded upon by Tim Pope](http://www.tpope.net/node/106).
 :	Maximum length of the commit message's subject line.  Defaults
-	to 65 if not specified or less than 0.
+	to 50 if not specified or less than 0.
 :	Maximum length of a line in the commit message's body.  Defaults
-	to 70 if not specified or less than 0.
+	to 72 if not specified or less than 0.
 :	Percentage of commit message lines allowed to exceed the