Trim trailing newline on cmdsubst when IFS=''

When $IFS is empty, command substitution no longer splits on newlines.
However we still want to trim off a single trailing newline, as most
commands will emit a trailing newline and it makes it harder to work
with their output.

exec.cpp

@@ -1540,7 +1540,7 @@ static int exec_subshell_internal(const wcstring &cmd, wcstring_list_t *lst, boo
     ASSERT_IS_MAIN_THREAD();
     int prev_subshell = is_subshell;
     const int prev_status = proc_get_last_status();
-    char sep=0;
+    bool split_output=false;
 
     //fprintf(stderr, "subcmd %ls\n", cmd.c_str());
 
@@ -1548,7 +1548,7 @@ static int exec_subshell_internal(const wcstring &cmd, wcstring_list_t *lst, boo
 
     if (! ifs.missing_or_empty())
     {
-        sep = '\n';
+        split_output=true;
     }
 
     is_subshell=1;
@@ -1580,11 +1580,13 @@ static int exec_subshell_internal(const wcstring &cmd, wcstring_list_t *lst, boo
     {
         const char *begin = io_buffer->out_buffer_ptr();
         const char *end = begin + io_buffer->out_buffer_size();
+        if (split_output)
+        {
             const char *cursor = begin;
             while (cursor < end)
             {
                 // Look for the next separator
-            const char *stop = (const char *)memchr(cursor, sep, end - cursor);
+                const char *stop = (const char *)memchr(cursor, '\n', end - cursor);
                 const bool hit_separator = (stop != NULL);
                 if (! hit_separator)
                 {
@@ -1599,6 +1601,17 @@ static int exec_subshell_internal(const wcstring &cmd, wcstring_list_t *lst, boo
                 cursor = stop + (hit_separator ? 1 : 0);
             }
         }
+        else
+        {
+            // we're not splitting output, but we still want to trim off a trailing newline
+            if (end != begin && end[-1] == '\n')
+            {
+                --end;
+            }
+            const wcstring wc = str2wcstring(begin, end - begin);
+            lst->push_back(wc);
+        }
+    }
 
     return subcommand_status;
 }

tests/read.in

@@ -7,6 +7,11 @@ set -l IFS \t
 count (echo one\ntwo)
 set -l IFS
 count (echo one\ntwo)
+echo [(echo -n one\ntwo)]
+count (echo one\ntwo\n)
+echo [(echo -n one\ntwo\n)]
+count (echo one\ntwo\n\n)
+echo [(echo -n one\ntwo\n\n)]
 set -le IFS
 
 function print_vars --no-scope-shadowing

tests/read.out

@@ -1,6 +1,15 @@
 2
 2
 1
+[one
+two]
+1
+[one
+two]
+1
+[one
+two
+]
 
 1 'hello' 1 'there'
 1 'hello there'