lint: use git ls-files for shellcheck (#4040)

Use `git ls-files` to generate the list of files to run through shellcheck.
In addition to a huge performance improvement (shellcheck linter runs 6-7x faster),
this ignores files in submodules and files specified in .gitignore.

Signed-off-by: Callahan Kovacs <callahan.kovacs@canonical.com>

setup.cfg

@@ -116,11 +116,8 @@ commands =
 description = Check spelling with shellcheck
 labels = lint
 skip_install = true
-allowlist_externals = bash
-commands =
-    bash -c "find . \( -name .git -o -name gradlew -o -name .tox -o -name .ruff_cache -o \
-    -path ./tests/spread/tools/snapd-testing-tools \) -prune -o -print0 | xargs -0 file -N \
-    | grep shell.script | cut -f1 -d: | xargs shellcheck"
+allowlist_externals = bash, git
+commands = bash -c "git ls-files | file --mime-type -Nnf- | grep shellscript | cut -f1 -d: | xargs shellcheck"
 
 [testenv:spread-shellcheck]
 description = Run shellcheck on spread's test.yaml files using spread-shellcheck.py

tests/spread/plugins/v1/gradle/snaps/gradlew-hello/gradlew

@@ -1,5 +1,7 @@
 #!/usr/bin/env bash
 
+# shellcheck disable=all
+
 ##############################################################################
 ##
 ##  Gradle start up script for UN*X