Fix pre-commit, prepare-commit-msg hooks

For users who had not overridden the `pre-commit` or
`prepare-commit-msg` Git hooks,
they would see `git commit` hang,
with nothing happening.

The command would exit without going to their editor.

The bug was introduced in d7f194f94e
and the fix is to return a 0 exit code when the override is not present.

git_template/hooks/pre-commit

@@ -1,4 +1,7 @@
 #!/bin/sh
 
 local_hook="$HOME"/.git_template.local/hooks/pre-commit
-[ -f "$local_hook" ] && . "$local_hook"
+
+if [ -f "$local_hook" ]; then
+  . "$local_hook"
+fi

git_template/hooks/prepare-commit-msg

@@ -1,4 +1,7 @@
 #!/bin/sh
 
 local_hook="$HOME"/.git_template.local/hooks/prepare-commit-msg
-[ -f "$local_hook" ] && . "$local_hook"
+
+if [ -f "$local_hook" ]; then
+  . "$local_hook"
+fi