fix(git): support setting executable bit

[ylemkimon]

Changes:

Support setting the executable bit (chmod +x) in git.commitFiles()

Context:

Sometimes, there is a need to set the executable bit in committed files, e.g., binary files.

This is a blocking issue for #11368.

The actual mode of the written file cannot be tested as:

• umask of test environments may vary
• some file systems including Windows don't support the mode

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

Windows and macOS tests: https://github.com/ylemkimon/renovate/actions/runs/1322981586

[rarkins]

I updated the workflows to add mac, windows. Should fail this one: https://github.com/ylemkimon/renovate/runs/3845610602?check_suite_focus=true

[viceice]

LGTM, but workflows need to be reverted.

[renovate-release]

🎉 This PR is included in version 27.31.4 🎉

The release is available on:

• GitHub release
• 27.31.4

Your semantic-release bot 📦🚀