fix(git-prompt): use `--git-common-dir` to load stash count (#13072)

plugins/git-prompt/gitstatus.py

@@ -23,9 +23,10 @@ def get_tagname_or_hash():
         return hash_
     return None
 
-# Re-use method from https://github.com/magicmonty/bash-git-prompt to get stashs count
+# Re-use method from https://github.com/magicmonty/bash-git-prompt to get stash count
+# Use `--git-common-dir` to avoid problems with git worktrees, which don't have individual stashes
 def get_stash():
-    cmd = Popen(['git', 'rev-parse', '--git-dir'], stdout=PIPE, stderr=PIPE)
+    cmd = Popen(['git', 'rev-parse', '--git-common-dir'], stdout=PIPE, stderr=PIPE)
     so, se = cmd.communicate()
     stash_file = '%s%s' % (so.decode('utf-8').rstrip(), '/logs/refs/stash')
 
@@ -35,7 +36,6 @@ def get_stash():
     except IOError:
         return 0
 
-
 # `git status --porcelain --branch` can collect all information
 # branch, remote_branch, untracked, staged, changed, conflicts, ahead, behind
 po = Popen(['git', 'status', '--porcelain', '--branch'], env=dict(os.environ, LANG="C"), stdout=PIPE, stderr=PIPE)