patch 9.1.0192: drop: don't rewind when editing the same file

Problem:  :drop tries to :rewind the argumentlist, which results in E37
          (after v9.1.0046)
Solution: instead of calling ex_rewind(), call open_buffer() only when
          re-using the initial empty buffer

fixes: #14219
closes: #14220

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/dumps/Test_drop_modified_1.dump b/src/testdir/dumps/Test_drop_modified_1.dump
new file mode 100644
index 0000000..bbd60c0
--- /dev/null
+++ b/src/testdir/dumps/Test_drop_modified_1.dump
@@ -0,0 +1,10 @@
+>T+0&#ffffff0|h|e| |q|u|i|c|k| |b|r|o|w|n| |f|o|x| |j|u|m|p|e|d| |o|v|e|r| |t|h|e| |l|a|z|y| |d|o|g|s| @29
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|:+0#0000000&|d|r|o|p| |X|d|r|o|p|_|m|o|d|i|f|i|e|d|.|t|x|t| @32|1|,|1| @10|A|l@1| 
diff --git a/src/testdir/test_excmd.vim b/src/testdir/test_excmd.vim
index 6a7829b..221ceb0 100644
--- a/src/testdir/test_excmd.vim
+++ b/src/testdir/test_excmd.vim
@@ -3,6 +3,7 @@
 source check.vim
 source shared.vim
 source term_util.vim
+source screendump.vim
 
 func Test_ex_delete()
   new
@@ -738,4 +739,19 @@
   call assert_fails(':--+foobar', 'E492:')
 endfunc
 
+func Test_drop_modified_file()
+  CheckScreendump
+  let lines =<< trim END
+  call setline(1, 'The quick brown fox jumped over the lazy dogs')
+  END
+  call writefile([''], 'Xdrop_modified.txt', 'D')
+  call writefile(lines, 'Xtest_drop_modified', 'D')
+  let buf = RunVimInTerminal('-S Xtest_drop_modified Xdrop_modified.txt', {'rows': 10,'columns': 40})
+  call term_sendkeys(buf, ":drop Xdrop_modified.txt\<CR>")
+  call VerifyScreenDump(buf, 'Test_drop_modified_1', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab