Always treat brace at command start as compound statement

For backwards compatibility, fish does not treat "{echo,hello}" as a compound
statement but as brace expansion (effectively "echo hello").  We interpret
"{X...}" as compound statement only if X is whitespace or ';' (which is an
interesting solution).

A brace expansion at the very start of a command 
is usually pointless (space separation is shorter).
The exception are cases where the command name and the first few arguments
share a suffix.

	$ {,1,2,3,4}echo
	1echo 2echo 3echo 4echo

Not sure if anyone uses anything like that.  Perhaps we want to trade
compatibility for simplicity. I don't have a strong opinion on this.

Always parse the opening brace as first character of a command token as
compound statement.
Brace expansion can still be used with a trick like: «''{echo,foo}»

Closes #11477

tests/checks/braces.fish

@@ -67,26 +67,33 @@ e{cho,cho,cho}
 { echo no semi }
 # CHECK: no semi
 
-# Ambiguous cases
+{echo no space}
+# CHECK: no space
 
+# Ambiguous case
 { echo ,comma;}
 # CHECK: ,comma
 
-PATH= {echo no space}
-# CHECKERR: fish: Unknown command: '{echo no space}'
-# CHECKERR: {{.*}}/braces.fish (line {{\d+}}):
-# CHECKERR: PATH= {echo no space}
-# CHECKERR:       ^~~~~~~~~~~~~~^
-
-PATH= {echo comma, no space;}
-# CHECKERR: fish: Unknown command: 'echo comma'
-# CHECKERR: {{.*}}/braces.fish (line {{\d+}}):
-# CHECKERR: PATH= {echo comma, no space;}
-# CHECKERR:       ^~~~~~~~~~~~~~~~~~~~~~^
+# Ambiguous case with no trailing space
+{echo comma, no space;}
+# CHECK: comma, no space
 
 # Ambiguous case with no space
-{echo,hello}
-# CHECK: hello
+PATH= {echo,hello}
+# CHECKERR: fish: Unknown command: echo,hello
+# CHECKERR: {{.*}}/braces.fish (line {{\d+}}):
+# CHECKERR: PATH= {echo,hello}
+# CHECKERR:        ^~~~~~~~~^
+
+function foo,
+    echo foo,
+end
+function bar
+    echo bar
+end
+{foo,;bar}
+# CHECK: foo,
+# CHECK: bar
 
 # Trailing tokens
 set -l fish (status fish-path)
@@ -157,7 +164,7 @@ end
 }
 # CHECK: while
 
-{ { echo inner}
+{{echo inner}
     echo outer}
 # CHECK: inner
 # CHECK: outer
@@ -172,9 +179,8 @@ complete foo -a '123 456'
 complete -C 'foo {' | sed 1q
 # CHECK: {{\{.*}}
 
-complete -C '{'
-echo nothing
-# CHECK: nothing
+complete -C '{' | grep ^if\t
+# CHECK: if{{\t}}Evaluate block if condition is true
 complete -C '{ ' | grep ^if\t
 # CHECK: if{{\t}}Evaluate block if condition is true
 

tests/checks/indent.fish

@@ -459,6 +459,9 @@ echo \\
     echo '{  {  }  }'
     # CHECK: { { } }
 
+    echo '{{}}'
+    # CHECK: { { } }
+
     echo '
 {