[clang-format][PR46043] Parse git config w/ implicit values

Summary:
https://bugs.llvm.org/show_bug.cgi?id=46043

Git's config is generally of the format 'key=val', but a setting
'key=true' can be written as just 'key'.  The git-clang-format script
expects a value and crashes in this case; this change handles implicit
'true' values in the script.

Reviewers: MyDeveloperDay, krasimir, sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80486