fix(changelog): fix for `${(@ps:$sep:)var}` construct in zsh < 5.0.8

In recent zsh versions, `${(@ps:$sep:)var}` where $sep is a variable containing
a separator string and $var is a string with multiple values separated by $sep,
the `p` flag makes zsh correctly expand $sep before splitting $var. In versions
older than 5.0.8, this doesn't happen, so we use `eval` to get the same effect.

tools/changelog.sh

@@ -428,7 +428,7 @@ function main {
     fi
 
     # Read the commit fields (@ is needed to keep empty values)
-    raw_fields=("${(@ps:$SEP:)raw_commit}")
+    eval "raw_fields=(\"\${(@ps:$SEP:)raw_commit}\")"
     hash="${raw_fields[1]}"
     refs="${raw_fields[2]}"
     subject="${raw_fields[3]}"