cmdutil: avoid wrapping ctx.phasestr() by _()
This wrapping seems meaningless, because:
- there is no "_()" invocation to prepare for extracting phase names
to be translated
"make update-pot" doesn't extract msgids for phase names
- phase names are kine of reserved keywords like as branch name "default"
repair: avoid string concatenation by + operator
String concatenation by "+" operator causes failure of extracting
messages to be translated.
Python automatically concatenates strings separated by whitespaces
into one string.