fix: use .gitignore for exludes

[tnijboer]

Fixes #2244.

Problem

Running copier update fails with OSError: [Errno 7] Argument list too long on projects with extensive .gitignore files or _skip_if_exists lists.

The cause is that copier builds the git apply command by adding an individual --exclude={pattern} argument for every exclusion. This exceeds the OS command-line argument length limit.

Solution

This patch refactors the exclusion handling for git apply:

1. All exclusion patterns (from .gitignore, all_skip_if_exists, answers_relpath) are consolidated into a single list.
2. This list is written to a temporary file.
3. The git apply command is modified to use a single --exclude-from=<temp_file> argument, pointing to the new file.
4. A try...finally block is used to guarantee the temporary file is cleaned up.

This change avoids the argument length limit and allows copier update to succeed on projects with many exclusions.

[sisp]

Several tests are failing; it seems that git apply does not have an --exclude-from flag. :thinking:

[tnijboer]

Yes, you're totally right. My bad, was confused with rsync. I updated the code to just check the .gitignore for folders, and if there are any files listed in git status --ignored --porcelain that are in an ignored folder just add the exclude for the folder instead so the list of excludes get way shorter.

[tnijboer]

@sisp what do you think of this approach?