Merge branch 'ar/diff-args-osx-precompose' into maint

Many commands normalize command line arguments from NFD to NFC
variant of UTF-8 on OSX, but commands in the "diff" family did
not, causing "git diff $path" to complain that no such path is
known to Git.  They have been taught to do the normalization.

* ar/diff-args-osx-precompose:
diff: run arguments through precompose_argv

diff --cc builtin/diff-tree.c
Simple merge

diff --cc builtin/diff.c
Simple merge