push: enhance unspecified push default warning

When the unset push.default warning message is displayed this may be
the first time many users encounter push.default.

Explain in the warning message in a compact manner what push.default
is and what the change means to the end-user to help the users decide.

Signed-off-by: Greg Jacobson <coder5000@gmail.com>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Helped-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Helped-by: Marc Branchaud <marcnarc@xiplink.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/push.c

@@ -174,6 +174,13 @@ N_("push.default is unset; its implicit value is changing in\n"
    "\n"
    "  git config --global push.default simple\n"
    "\n"
+   "When push.default is set to 'matching', git will push local branches\n"
+   "to the remote branches that already exist with the same name.\n"
+   "\n"
+   "In Git 2.0, Git will default to the more conservative 'simple'\n"
+   "behavior, which only pushes the current branch to the corresponding\n"
+   "remote branch that 'git pull' uses to update the current branch.\n"
+   "\n"
    "See 'git help config' and search for 'push.default' for further information.\n"
    "(the 'simple' mode was introduced in Git 1.7.11. Use the similar mode\n"
    "'current' instead of 'simple' if you sometimes use older versions of Git)");