Default autogen.sh to 'all'

This addresses a problem noted by RMS in:
http://lists.gnu.org/archive/html/emacs-devel/2017-08/msg00052.html
* autogen.sh (do_git): Set to true if this script is invoked
with no arguments and there is a .git subdirectory.

CONTRIBUTE

@@ -14,7 +14,7 @@ Briefly, the following shell commands build and run Emacs from scratch:
 	git config --global transfer.fsckObjects true
 	git clone git://git.sv.gnu.org/emacs.git
 	cd emacs
-	./autogen.sh all
+	./autogen.sh
 	./configure
 	make
 	src/emacs

INSTALL.REPO

@@ -19,7 +19,7 @@ To use the autotools, run the following shell command to generate the
 'configure' script and some related files, and to set up your git
 configuration:
 
-  $ ./autogen.sh all
+  $ ./autogen.sh
 
 You can then configure your build as follows:
 

autogen.sh

@@ -127,7 +127,8 @@ done
 
 case $do_autoconf,$do_git in
   false,false)
-    do_autoconf=true;;
+    do_autoconf=true
+    test -e .git && do_git=true;;
 esac
 
 # Generate Autoconf-related files, if requested.