[oil-language] Change the default for echo -sep to '\n'.

The default is to generate "testdata", i.e. this makes sense:

echo 3 1 2 | sort -n

But -sep '' and -sep ' ' are expected to be common invocations.

doc/oil-manual.md

@@ -234,9 +234,8 @@ var s = sorted(ARGV)
 `shopt -s simple_echo` changes the `echo` builtin to accept the following long
 flags, as well as the `--` separator between flags and args.
 
-- `-sep`: Characters to separate args to `echo`.  (Default: empty string)
-- `-end`: Characters to terminate the whole `echo` invocation.  (Default:
-  newline)
+- `-sep`: Characters to separate each argument.  (Default: newline)
+- `-end`: Characters to terminate the whole invocation.  (Default: newline)
 - `-n`: A synonym for `-end ''`.
 
 #### push

osh/builtin_pure.py

@@ -510,12 +510,12 @@ def __call__(self, arg_vec):
 ECHO_SPEC.ShortFlag('-n')
 
 OIL_ECHO_SPEC = args.OilFlags()
-OIL_ECHO_SPEC.Flag('-sep', args.Str, default='',
-                    help='Characters to separate args')
+OIL_ECHO_SPEC.Flag('-sep', args.Str, default='\n',
+                    help='Characters to separate each argument')
 OIL_ECHO_SPEC.Flag('-end', args.Str, default='\n',
-                    help='Characters to terminate the echo invocation')
+                    help='Characters to terminate the whole invocation')
 OIL_ECHO_SPEC.Flag('-n', args.Bool, default=False,
-                    help='Omit newline')
+                    help="Omit newline (synonym for -end '')")
 
 
 class Echo(object):

spec/builtin-oil.test.sh

@@ -22,7 +22,12 @@ argv.py @a
 shopt -s all:oil
 var a = @('a b' 'c d')
 echo @a
+echo .
 echo -- @a
+echo .
+
+echo -sep '' -end '' @a; echo
+echo .
 
 echo -sep '_' -- @a
 echo -sep '_' -end $' END\n' -- @a
@@ -33,9 +38,16 @@ echo --sep '_' --end $' END\n' -- @a
 echo -n x
 echo -n y
 echo
+
 ## STDOUT:
+a b
+c d
+.
+a b
+c d
+.
 a bc d
-a bc d
+.
 a b_c d
 a b_c d END
 a b_c d END