git: Add diff drivers for function context

Map common source and documentation file types to Git's built-in diff
drivers so `git diff -W` expands hunks using language-aware function or
section boundaries instead of generic text context.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

.gitattributes

@@ -1 +1,8 @@
 Dockerfile* linguist-language=Dockerfile
+
+*.c    diff=cpp
+*.h    diff=cpp
+*.go   diff=golang
+*.md   diff=markdown
+*.py   diff=python
+*.sh   diff=bash