Provide a GitHub PR title from `.git/info/title` file

[russellyeo]

Similar to the functionality for adding a PR description from .git/info/description, it would be useful to be able to set the PR title from a .git/info/title file.

This is a suggestion for an alternative method to the --title CLI parameter, which should take precedence over this method.

Expected behavior

Given that

• .git/info/title contains a title, e.g. "Add New Feature"
• .git/info/description contains a description, e.g. "# Summary\n..."

When we invoke

git machete github create-pr

Then a pull request is created with the title and description set.