survey: turn into a thin shim over git repo structure

`git survey` was an experimental scale-measurement tool whose
distinctive features (ref-kind filters, top-N path tables) are now
all available in `git repo structure`. With the path-level reporting
in place (commits "repo: filter the structure scope via
--ref-filter=<pattern>" and "repo: report top-N paths by count, disk,
and inflated size in structure"), there is no functionality `git
survey` provides that `git repo structure` cannot.

Replace the 764-line `git survey` implementation with a roughly
hundred-line shim that:

  * Accepts the existing `git survey` command line so callers in
    scripts continue to parse without changes.
  * Emits a deprecation warning naming the replacement command, so
    interactive users learn about the migration target.
  * Translates the survey-specific knobs into the equivalent
    `git repo structure` invocation and re-execs the canonical
    command via `execv_git_cmd()`. Per-kind ref selectors fan out
    into the corresponding `refs/heads/*`, `refs/tags/*`, etc.
    `--ref-filter` patterns; `--top=<N>` is forwarded directly;
    `--all-refs` becomes the absence of any `--ref-filter`.

Two survey options have no `git repo structure` counterpart:
`--verbose` controlled per-step trace output the new command does
not emit, and `--detached` selected the detached HEAD which
`git repo structure` does not enumerate separately. Both are
silently accepted and produce a single warning each, so old
invocations keep working while the absence of these knobs in `git
repo structure` is made visible.

Rewrite t8100 to assert the shim's contract: the deprecation
warning is printed, the output is byte-identical to a corresponding
`git repo structure` invocation, and the per-kind selector
translation produces the right `--ref-filter` pattern. The
preceding survey-specific output assertions (the multi-column
plaintext tables) no longer apply, since `git repo structure`'s
output format is now the canonical one and is covered by t1901.

The `survey.*` configuration keys (`survey.top`, `survey.progress`,
`survey.verbose`) are no longer honored by the shim. They were
mirrored by the preceding `repo.structure.top` work for the most
useful knob; users with `survey.top` set in config should migrate
to `repo.structure.top`. This is a backward-incompatible removal
documented by the deprecation notice in `git-survey.adoc`.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

Documentation/git-survey.adoc

@@ -17,8 +17,8 @@ NOTE: `git survey` is being superseded by `git repo structure`. New
 deployments and new features should use `git repo structure`; its
 `--ref-filter=<pattern>` option subsumes the various `--branches`,
 `--tags`, and `--remotes` flags here, and `--top=<N>` provides the
-same detail tables. A future release will turn `git survey` into a
-thin shim over `git repo structure`. See linkgit:git-repo[1].
+same detail tables. During the deprecation phase, `git survey` is
+a thin shim over `git repo structure`. See linkgit:git-repo[1].
 
 Survey the repository and measure various dimensions of scale.