Merge branch 'jk/ref-symlink-loop'

A stray symbolic link in $GIT_DIR/refs/ directory could make name
resolution loop forever, which has been corrected.

* jk/ref-symlink-loop:
files_read_raw_ref: prevent infinite retry loops in general
files_read_raw_ref: avoid infinite loop on broken symlinks

diff --cc refs/files-backend.c
Simple merge