Merge branch 'ar/add-unreadable'

* ar/add-unreadable:
Add a config option to ignore errors for git-add
Add a test for git-add --ignore-errors
Add --ignore-errors to git-add to allow it to skip files with read errors
Extend interface of add_files_to_cache to allow ignore indexing errors
Make the exit code of add_file_to_index actually useful

diff --cc builtin-commit.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc t/t3700-add.sh
Simple merge