git: Disable log.showSignature for internal commands (#55708)

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #53604.

When `log.showSignature = true`, git prepends signature verification
lines to stdout before the `--format` output. The null-separated parsers
in `crates/git/src/repository.rs` weren't expecting that, so SHAs ended
up corrupted — the git graph detail panel showed "0 changed files" for
every commit, and file history was similarly broken.

Setting `-c log.showSignature=false` in `build_command` is the same
trick we already use for `core.fsmonitor`. It only affects
log/show/whatchanged, so other commands aren't touched.

Verified locally with an SSH-signed repo: before this change the detail
panel said "0 changed files"; after, the modified files show up
correctly.

Release Notes:

- git_graph: Fix breakage that occurs when `log.showSignature` is
enabled

---------

Co-authored-by: Anthony Eid <anthony@zed.dev>

crates/git/src/repository.rs

@@ -3415,8 +3415,12 @@ impl GitBinary {
     {
         let mut command = new_command(&self.git_binary_path);
         command.current_dir(&self.working_directory);
+        // Disabled to stop malicious actors from running arbitrary commands via fsmonitor hooks
         command.args(["-c", "core.fsmonitor=false"]);
+        // Prepended signature lines would corrupt our --format parsers.
+        command.args(["-c", "log.showSignature=false"]);
         command.arg("--no-optional-locks");
+        // Internal commands must be non-interactive so background tasks never block on user input.
         command.arg("--no-pager");
 
         if !self.is_trusted {
@@ -3814,6 +3818,51 @@ mod tests {
         );
     }
 
+    #[gpui::test]
+    async fn test_build_command_disables_log_show_signature(cx: &mut TestAppContext) {
+        cx.executor().allow_parking();
+        let dir = tempfile::tempdir().unwrap();
+        git2::Repository::init(dir.path()).unwrap();
+
+        let git = GitBinary::new(
+            PathBuf::from("git"),
+            dir.path().to_path_buf(),
+            dir.path().join(".git"),
+            cx.executor(),
+            true,
+        );
+        let output = git
+            .build_command(&["config", "--get", "log.showSignature"])
+            .output()
+            .await
+            .expect("git config should run");
+        let stdout = String::from_utf8_lossy(&output.stdout);
+        assert_eq!(
+            stdout.trim(),
+            "false",
+            "log.showSignature should be disabled for trusted repos"
+        );
+
+        let git = GitBinary::new(
+            PathBuf::from("git"),
+            dir.path().to_path_buf(),
+            dir.path().join(".git"),
+            cx.executor(),
+            false,
+        );
+        let output = git
+            .build_command(&["config", "--get", "log.showSignature"])
+            .output()
+            .await
+            .expect("git config should run");
+        let stdout = String::from_utf8_lossy(&output.stdout);
+        assert_eq!(
+            stdout.trim(),
+            "false",
+            "log.showSignature should be disabled for untrusted repos"
+        );
+    }
+
     #[gpui::test]
     async fn test_path_for_index_id_uses_real_git_directory(cx: &mut TestAppContext) {
         cx.executor().allow_parking();