Fix stash deletion failing when trash is disabled

backupStash() called Trash.newFile(), which raises Trash.BackupSkipped
when the trash is disabled (maxTrashFiles == 0). backupStash did not
catch it, so DropStash and 'apply and delete' aborted with an error and
the stash was never removed. The dead 'if not trashFile' guard assumed
newFile() returns None, which it never does.

Catch Trash.BackupSkipped and skip the backup, matching how DiscardFiles
handles the same exception.

Author: grub-basket

gitfourchette/tasks/stashtasks.py

@@ -5,6 +5,7 @@
 # -----------------------------------------------------------------------------
 
 import itertools
+import logging
 
 from gitfourchette.forms.stashdialog import StashDialog
 from gitfourchette.localization import *
@@ -15,6 +16,8 @@
 from gitfourchette.toolbox import *
 from gitfourchette.trash import Trash
 
+logger = logging.getLogger(__name__)
+
 _indexStatusTable = {
     "A": FileStatus.INDEX_NEW,
     "D": FileStatus.INDEX_DELETED,
@@ -40,9 +43,10 @@
 
 
 def backupStash(repo: Repo, stashCommitId: Oid):
-    trashFile = Trash.instance().newFile(repo.workdir, ext=".txt", originalPath="DELETED_STASH")
-
-    if not trashFile:
+    try:
+        trashFile = Trash.instance().newFile(repo.workdir, ext=".txt", originalPath="DELETED_STASH")
+    except Trash.BackupSkipped as ex:
+        logger.warning(f"Stash backup skipped: {ex}")
         return
 
     text = F"""\