SCM_CURRENT_USER supports accented characters

`tr 'A-Z' 'a-z'` will only convert non-accented characters. Switching to
`[:upper:]` and `[:lower:]` adds support for accents.
See https://github.com/koalaman/shellcheck/wiki/SC2018

Additionally, printf's character splitting does not support accented
characters, so output isn't as expected. Using bash's variable expansion
syntax instead will correctly get the full accented character.

test/themes/base.theme.git.bats

@@ -362,6 +362,11 @@ setup_repo_with_upstream() {
   git_prompt_vars
   assert_equal "$SCM_BRANCH" "${pre} ☺︎ cu"
 
+  git config user.name "Çool Üser"
+
+  git_prompt_vars
+  assert_equal "$SCM_BRANCH" "${pre} ☺︎ çü"
+
   # show initials set by `git pair`
 
   git config user.initials "ab cd"