Merge branch 'ds/completion-silence-in-tree-path-probe'

Merge branch 'ds/completion-silence-in-tree-path-probe'

An internal ls-tree call made by completion code only to probe if
a path exists in the tree recorded in a commit object leaked error
messages when the path is not there.  It is not an error at all and
should not be shown to the end user.

* ds/completion-silence-in-tree-path-probe:
  git-completion.bash: silence "not a valid object" errors