base: Honor .gitignore for rsync

I am guessing I may be one of the first people to try using tmt
with a Rust project. The default for the `cargo` toolchain
is to keep a *lot* of cached incremental data in `target/`.
In my case with bootc, it's currently 20G.

A plain rsync() of this is *incredibly* inefficient. rsync doesn't
even use reflinks if available, though that's a distinct bug.

Use `git ls-files` to honor `.gitignore`.

Signed-off-by: Colin Walters <walters@verbum.org>

tmt/base.py

@@ -8,6 +8,7 @@
 import os
 import re
 import shutil
+import subprocess
 import sys
 import tempfile
 import time
@@ -1806,7 +1807,14 @@ def _initialize_worktree(self) -> None:
 
         # Sync metadata root to the worktree
         self.debug(f"Sync the worktree to '{self.worktree}'.", level=2)
-        self.run(Command("rsync", "-ar", "--exclude", ".git", f"{tree_root}/", self.worktree))
+        excludes_tempfile = tempfile.NamedTemporaryFile()
+        # If we're in a git repository, honor .gitignore; xref
+        # https://stackoverflow.com/questions/13713101/rsync-exclude-according-to-gitignore-hgignore-svnignore-like-filter-c
+        if os.path.isdir(f"{tree_root}/.git"):
+            subprocess.check_call(["git", "ls-files", "--exclude-standard", "-oi", "--directory"], stdout=excludes_tempfile)
+        # Note: rsync doesn't use reflinks right now, so in the future it'd be even better to
+        # use e.g. `cp` but filtering out the above.
+        self.run(Command("rsync", "-ar", "--exclude", ".git", "--exclude-from", excludes_tempfile.name, f"{tree_root}/", self.worktree))
 
     def _initialize_data_directory(self) -> None:
         """