[Bug] fetch --deepen truncates history in v2.54.0

Repeatedy called `git fetch --deepen 2` inside a shallow repo that was a file:// clone of another repo. Once all commits had been fetched, a subsequent `fetch --deepen` appears to "reset" the repo back to being shallow with a depth of 2. A reproduction script is included below. This issue appears to have been introduced in v2.54.0.

I expected `git fetch --deepen` in a non-shallow repo with no upstream commits to be a no-op.

`git log` history is truncated to two commits, and repo is considered shallow by `git rev-parse --is-shallow-repository`.

The previously-present commits in `git log` are missing, and the repo is again considered shallow.

Commit 3ef68ff seems relevant.

The following script reproduces the issue in 2.54.0, and does not reproduce the issue in 2.53.0:

``` mkdir repro.git cd repro.git

git init

for i in $(seq 1 4); do
  echo "$i" >> file.txt
  git add file.txt
  git commit -m "Change $i"
done

cd ..

git clone --depth 2 "file://$PWD/repro.git" repro_clone.git cd repro_clone.git

echo "Shallow repo? $(git rev-parse --is-shallow-repository)" git log --oneline

for i in $(seq 1 3); do
  git fetch --deepen 2
  echo "Shallow repo? $(git rev-parse --is-shallow-repository)"
  git log --oneline
done
```

The key lines in the output are: ``` Shallow repo? true 63d1ebe (HEAD -> master, origin/master, origin/HEAD) Change 4 864e13c (grafted) Change 3 remote: Enumerating objects: 10, done. remote: Counting objects: 100% (10/10), done. remote: Compressing objects: 100% (2/2), done. remote: Total 6 (delta 1), reused 0 (delta 0), pack-reused 0 (from 0) Unpacking objects: 100% (6/6), 351 bytes | 175.00 KiB/s, done.

Shallow repo? true 63d1ebe (HEAD -> master, origin/master, origin/HEAD) Change 4 864e13c Change 3 3e05d14 Change 2 1d9fe14 (grafted) Change 1 remote: Total 0 (delta 0), reused 0 (delta 0), pack-reused 0 (from 0)

Shallow repo? false 63d1ebe (HEAD -> master, origin/master, origin/HEAD) Change 4 864e13c Change 3 3e05d14 Change 2 1d9fe14 Change 1 remote: Total 0 (delta 0), reused 0 (delta 0), pack-reused 0 (from 0)

Shallow repo? true 63d1ebe (HEAD -> master, origin/master, origin/HEAD) Change 4 864e13c (grafted) Change 351 ```

N.b. that 1d9fe14 was present after the second iteration but missing after the third, along with `--is-shallow-repository` changing from false back to true.

[System Info]
git version:
git version 2.54.0
cpu: arm64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
rust: disabled
feature: fsmonitor--daemon
gettext: enabled
libcurl: 8.7.1
zlib: 1.2.12
SHA-1: SHA1_DC
SHA-256: SHA256_BLK
default-ref-format: files
default-hash: sha1
uname: Darwin 25.4.0 Darwin Kernel Version 25.4.0: Thu Mar 19 19:33:25
PDT 2026; root:xnu-12377.101.15~1/RELEASE_ARM64_T6041 arm64
compiler info: clang: 21.0.0 (clang-2100.0.123.102)
libc info: no libc information available
$SHELL (typically, interactive shell): /bin/zsh