[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.

bootstrap/packages.txt

@@ -15,6 +15,7 @@ bison
 chromium
 dhcpcd
 dialog
+diff-so-fancy
 docker
 docx2txt
 dunst