[git] Add diff-so-fancy as git diff

Added package diff-so-fancy and make diff use it by default for its
diffs by setting it as its diff-pager.

Added sanity function dsf which invokes diff-so-fancy if it exists or
degrades gracefully if it does not. Use this function for all git diff
needs.

Pre-fill the pager search term in git to enable jumping between changed
files with n/N.

git/.local/bin/dsf

@@ -0,0 +1,18 @@
+#!/usr/bin/env sh
+# Pretty git diff display
+
+# degrade diffs gracefully:
+# prefer diff-so-fancy if it is installed,
+# otherwise fallback to git's own prettifier,
+# if nothing exists just pass it on
+dsf() {
+  if exist diff-so-fancy; then
+    diff-so-fancy
+  elif exist /usr/share/git/diff-highlight/diff-highlight; then
+    /usr/share/git/diff-highlight/diff-highlight
+  else
+    cat
+  fi
+}
+
+dsf