Add function to update a Git repository

Should work like:

    $ git remote update

From what I can tell from:
https://github.com/git/git/blob/a0dda6023ed82b927fa205c474654699a5b07a82/builtin/remote.c#L1452-L1490
this translates to something like:

    $ git fetch --prune --multiple default --all

1 files changed, 6 insertions, 2 deletions

diff --git a/src/main.rs b/src/main.rs
index e31da8e..70489bb 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -8,8 +8,12 @@ fn main() {
     //
     // dbg!(&repos);
 
-    git::mirror(
-        "https://github.com/teddywing/google-calendar-rsvp.git",
+    // git::mirror(
+    //     "https://github.com/teddywing/google-calendar-rsvp.git",
+    //     Path::new("/tmp/grsvp"),
+    // ).unwrap();
+
+    git::update(
         Path::new("/tmp/grsvp"),
     ).unwrap();
 }