fix: separate fallback push branch from pathspecs

[lawrence3699]

Closes #1395

Context

When @{push} is unavailable, PushFiles() falls back to git diff --name-only HEAD <default-branch>.
If the repository also contains a directory with the same name as that default branch, Git treats the branch name as ambiguous and the pre-push hook fails before it can resolve {push_files}.

Changes

• terminate the fallback git diff revision list with --
• add regression coverage for the branch/path collision case

Validation

• go test ./internal/git -run TestPushFiles -count=1
• go test ./internal/git -count=1
• go test ./internal/run/controller/... -count=1
• go test ./... -count=1
• manual repro on a throwaway repo where origin/HEAD points to dev and the worktree contains dev/

[Copilot]

Pull request overview

This PR fixes a Git argument ambiguity in Repository.PushFiles() when falling back from @{push} to a default branch name that also exists as a path in the worktree, by separating revisions from pathspecs using -- and adding a regression test for that scenario.

Changes:

• Update the fallback git diff --name-only HEAD <branch> invocation to git diff --name-only HEAD <branch> -- to disambiguate branch vs. path names.
• Add a regression test covering the branch/path collision case (e.g., default branch dev and a dev/ directory present).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/git/repository.go	Appends -- to the fallback git diff command to prevent branch/path ambiguity errors.
internal/git/repository_test.go	Adds a test ensuring the fallback diff uses -- and succeeds when a directory matches the branch name.

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[mrexox]

Thank you for preparing the PR! I'll fix linter issues