Merge branch 'jk/read-in-full'

Code clean-up to prevent future mistakes by copying and pasting
code that checks the result of read_in_full() function.

* jk/read-in-full:
worktree: check the result of read_in_full()
worktree: use xsize_t to access file size
distinguish error versus short read from read_in_full()
avoid looking at errno for short read_in_full() returns
prefer "!=" when checking read_in_full() result
notes-merge: drop dead zero-write code
files-backend: prefer "0" for write_in_full() error check

diff --cc packfile.c
Simple merge

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

diff --cc sha1_file.c
Simple merge