tree: Fix various typos and outdated terminology

This was mostly machine generated with the following command:
```
codespell \
    --builtin clear,rare,usage \
    --skip './po/*' --skip './.git/*' --skip './NEWS*' \
    --write-changes .
```
using the latest git version of `codespell` as per [these
instructions](https://github.com/codespell-project/codespell#user-content-updating).

Then I manually checked each change using `git add -p`, made a few
manual fixups and dropped a load of incorrect changes.

There are still some outdated or loaded terms used in GLib, mostly to do
with git branch terminology. They will need to be changed later as part
of a wider migration of git terminology.

If I’ve missed anything, please file an issue!

Signed-off-by: Philip Withnall <withnall@endlessm.com>

gobject/tests/threadtests.c

@@ -94,7 +94,7 @@ tester_init_thread (gpointer data)
 {
   const GInterfaceInfo face2_interface_info = { (GInterfaceInitFunc) interface_per_class_init, NULL, NULL };
   gpointer klass;
-  /* first, syncronize with other threads,
+  /* first, synchronize with other threads,
    * then run interface and class initializers,
    * using unsafe_call_counter concurrently
    */