### Please describe the problem.

[[!meta title="import tree creates empty commits"]]

Adb special remote on both "import" and "sync" commands creates empty commits in "main" branch,
even if files hadn't changed  in either remote or local repo.

This clutters history of "main" branch (which is mostly manually curated),
and hides commits actually introducing added/deleted files amongst the clutter.

Moreover, this behavior differs from expectations: "sync" with proper remote never created empty commits in "main" branch.

## What steps will reproduce the problem?

Setup adb import/export, like here: 
[[https://git-annex.branchable.com/forum/Mixed_content_repos_with_import_and_export/#comment-764ac971faf756140055333649ffb94c]].

Repetition of "git annex sync --content" introduces new empty commits without proper reason.

### What version of git-annex are you using? On what operating system?
git-annex version: 8.20211118-g23ee48898


### Please provide any additional information below.

Example of "main" branch history cluttered by empty commits:

[[!format txt """
*   2022-01-01 bb23200 remote tracking branch                                       (android/main) [annex]
|\
* | 2022-01-01 42b8111 remote tracking branch                                       (HEAD -> main, synced/main) [annex]
|\|
| * 2022-01-01 223f5f7 import from android                                          [annex]
* | 2022-01-01 f5b5f0a remote tracking branch                                       [annex]
|\|
* | 2022-01-01 2a6177e remote tracking branch                                       [annex]
|\|
| * 2022-01-01 7ef4649 import from android                                          [annex]
"""]]


