Add file completion support for bash completion

[williamthome]

Summary

This PR enhances bash completion for rebar3 by adding file completion support for commands that accept file arguments and fixing completion behavior issues.

Changes

• File completion for test commands: ct and eunit now complete .erl files and directories
• File completion for utility commands: completion, shell, and tar now complete files and directories
• Fixed escaping issues: Removed problematic space-appending logic that caused unwanted backslashes
• Improved completion options: Changed from -o nospace to -o filenames for better file handling
• Added test coverage: New check_bash_file_completion test validates file completion functionality in CI

Examples

Before (no file completion):

$ rebar3 ct --suite test/<Tab>
# No completion - user had to type full paths manually

$ rebar3 eunit --dir <Tab>
# Only showed flags, no test files

After (with file completion):

$ rebar3 ct --suite test/<Tab>
test/my_app_SUITE.erl  test/helper_tests.erl  test/integration/

$ rebar3 eunit --dir <Tab>
test/unit_tests.erl  src/my_module.erl  test/

$ rebar3 completion --file <Tab>
completion.bash  scripts/  config/

$ rebar3 shell --env-file <Tab>
.env  config.sys  scripts/

Escaping fix:

# Before: unwanted backslashes
$ rebar3 ct --su<Tab> → $ rebar3 ct --suite\ 

# After: clean completion
$ rebar3 ct --su<Tab> → $ rebar3 ct --suite 

Testing

To test these changes:

1. Build and install the updated rebar3:

   ./bootstrap
   ./rebar3 local install

2. Generate the new completion file:

   ./rebar3 completion --file completion.bash

3. Source the completion in a new terminal:

   source _build/default/completion.bash

4. Test file completion:

   # In a project with test files
   rebar3 ct --suite test/<Tab>
   rebar3 eunit --dir <Tab>
   rebar3 completion --file <Tab>
   rebar3 shell --env-file <Tab>

5. Run the test suite:

./rebar3 ct --suite apps/rebar/test/rebar_completion_SUITE

The test automatically runs in CI and verifies:

• File completion logic is present for ct and eunit commands
• .erl file filtering works correctly
• Directory completion is included
• Other file-accepting commands (completion, shell, tar) are handled
• Fallback completion logic is preserved

Backward Compatibility

This change is backward compatible - existing completion behavior is preserved while adding new file completion capabilities.