bash completion: add space between branch name and status flags

bash completion: add space between branch name and status flags

Improve the readability of the bash prompt by adding a space between
the branch name and the status flags (dirty, stash, untracked).

While we are cleaning up this section of code, the two cases for
formatting the prompt are identical except for the format string,
so make them the same.

Suggested-by: Roman Fietze <redacted>
Signed-off-by: Shawn O. Pearce <redacted>
Signed-off-by: Junio C Hamano <redacted>