Fix --version output

[timvaillancourt]

Description

This PR fixes a problem introduced in #1347 due to my forgetting to add newline character to fmt.Printf(...) 🤦

Secondly, I've made an empty GitCommit display a bit more clearer

Before (notice no \n char)

tvaillancourt@tvailla-ltmxctu gh-ost % ./gh-ost --version
unversioned (git commit: )%  

After: undefined AppVersion+GitCommit

tvaillancourt@tvailla-ltmxctu gh-ost % ./gh-ost --version                        
unversioned (git commit: unknown)
tvaillancourt@tvailla-ltmxctu gh-ost %

After

tvaillancourt@tvailla-ltmxctu gh-ost % go build -o gh-ost -ldflags "-X main.AppVersion=1.1.6 -X main.GitCommit=$(git rev-parse HEAD)" ./go/cmd/gh-ost/main.go
tvaillancourt@tvailla-ltmxctu gh-ost % ./gh-ost --version                                                                                                    
1.1.6 (git commit: 5ebb95347e2002a5a959483f3e801cd3267e7e88)
tvaillancourt@tvailla-ltmxctu gh-ost %

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[timvaillancourt]

Let's always fallback to unversioned/unknown so that the starting gh-ost %+v logline later also gets this fallback instead of ""