Compatibility with libgit2 0.27

git_config_add_file_ondisk() now takes an additional (optional) 'repo' argument. Fix the build failure.

[smcv: Make the API change conditional on LIBGIT2_SOVERSION so we can build against either version]

Fixes: #1 (closed)

Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=903146

Signed-off-by: Simon McVittie smcv@debian.org