argparse: Fix validation for short-only-flags

This read something like `o=!_validate_int`, and the flag modifier
reading kept the pointer after the `!`, so it created a long flag
called `_validate_int`, which meant it would not only error out form

```fish
argparse 'i=!_validate_int' 'o=!_validate_int' -- $argv
```

with "Long flag '_validate_int' already defined", but also set
$_flag_validate_int.

Fixes #5864.

src/builtin_argparse.cpp

@@ -181,6 +181,8 @@ static bool parse_flag_modifiers(const argparse_cmd_opts_t &opts, const option_s
     if (*s == L'!') {
         s++;
         opt_spec->validation_command = wcstring(s);
+        // Move cursor to the end so we don't expect a long flag.
+        while (*s) s++;
     } else if (*s) {
         streams.err.append_format(BUILTIN_ERR_INVALID_OPT_SPEC, opts.name.c_str(),
                                   option_spec.c_str(), *s);

tests/argparse.err

@@ -121,3 +121,8 @@ in function 'notargparse'
 
 ####################
 # Checking arguments after --
+
+####################
+# Checking validation for short flags only
+argparse: Value 'banana' for flag 'o' is not an integer
+argparse: Value '-o' for flag 'i' is not an integer

tests/argparse.in

@@ -195,3 +195,12 @@ begin
     argparse a/alpha -- a --alpha -- b -a
     printf '%s\n' $argv
 end
+
+# #5864 - short flag only with same validation function.
+logmsg Checking validation for short flags only
+argparse 'i=!_validate_int' 'o=!_validate_int' -- -i 2 -o banana
+argparse 'i=!_validate_int' 'o=!_validate_int' -- -i -o banana
+begin
+    argparse 'i=!_validate_int' 'o=!_validate_int' -- -i 2 -o 3
+    set -l
+end

tests/argparse.out

@@ -146,3 +146,15 @@ alpha aaaa
 a
 b
 -a
+
+####################
+# Checking validation for short flags only
+_flag_a 'alpha'  'aaaa'
+_flag_b -b
+_flag_break -b
+_flag_i 2
+_flag_m 1
+_flag_max 1
+_flag_o 3
+argv
+saved_status 57