Merge branch 'ar/unlink-err'

* ar/unlink-err:
print unlink(2) errno in copy_or_link_directory
replace direct calls to unlink(2) with unlink_or_warn
Introduce an unlink(2) wrapper which gives warning if unlink failed

diff --cc builtin-clone.c
Simple merge

diff --cc builtin-fetch-pack.c
Simple merge

diff --cc diff.c
Simple merge

diff --cc fast-import.c
Simple merge

diff --cc lockfile.c
Simple merge

diff --cc server-info.c
Simple merge

diff --cc sha1_file.c
Simple merge

diff --cc transport.c
Simple merge