[git-llvm] Use --force-interactive when commiting to enable SVN to prompt password

When svn does not know the password and it has to prompt, it needs to query.
However it won't when invoked from the Python script and instead fails with:

svn: E215004: Authentication failed and interactive prompting is disabled; see the --force-interactive option

Differential Revision: https://reviews.llvm.org/D27274

llvm-svn: 288266