Merge branch 'jk/diagnose-config-mmap-failure' into maint

The configuration reader/writer uses mmap(2) interface to access
the files; when we find a directory, it barfed with "Out of memory?".

* jk/diagnose-config-mmap-failure:
xmmap(): drop "Out of memory?"
config.c: rewrite ENODEV into EISDIR when mmap fails
config.c: avoid xmmap error messages
config.c: fix mmap leak when writing config
read-cache.c: drop PROT_WRITE from mmap of index

diff --cc config.c
Simple merge

diff --cc git-compat-util.h
Simple merge

diff --cc read-cache.c
Simple merge

diff --cc sha1_file.c
Simple merge