tree: Fix various typos and outdated terminology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There are still some outdated or loaded terms used in GLib, mostly to do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>

glib/grand.c

@@ -582,7 +582,7 @@ gdouble
 g_rand_double (GRand *rand)
 {    
   /* We set all 52 bits after the point for this, not only the first
-     32. Thats why we need two calls to g_rand_int */
+     32. That's why we need two calls to g_rand_int */
   gdouble retval = g_rand_int (rand) * G_RAND_DOUBLE_TRANSFORM;
   retval = (retval + g_rand_int (rand)) * G_RAND_DOUBLE_TRANSFORM;