Use svn_hash_gets

See attached patch. Any objections to doing that (and similar changes
to the rest of the code)?

---
This was generated by:
1. 'spatch -sp_file 1 subversion/libsvn_wc/*.c', with the following patch:
[[[
svn-qavm,0:svn/t1% cat 1.cocci 
@@
expression ht, key;
@@
-apr_hash_get(ht, key, APR_HASH_KEY_STRING)
+svn_hash_gets(ht, key)
 
@@
expression ht, key, val;
@@
-apr_hash_set(ht, key, APR_HASH_KEY_STRING, val)
+svn_hash_sets(ht, key, val)
 
]]]
2. vim, with
:set aw et ai si ci sw=2 ts=2 cin cino=>2,n2,f0,{2,}0,=2,t0,+4,C1,(0,u0,*300
:map <F3> :cn<CR>n
:map <F4> gw%
:map <F5> f,f<Space>r<CR>gw%
:grep -R svn_hash_sets subversion/
:" ### TODO: that grep should be improved to match only lines over 80 chars.
(those cinoptions don't format blocks correctly, but they suffice for
the one-line task at hand here)