Fix #5218, Compile age in banner

[ivarne]

After static compillation of base Julia was implemented, the startup
banner showed the age of the newest commit in (origin/)master at the
time of compillation, instead of at startup time. This commit moves the
age calculation into the Base.banner() function, and moves the function
from base/client.jl to base/version.jl in order to have access to the
right variables.

CC: @staticfloat

[staticfloat]

I like the idea, but I think we can do better than search/replacing a magic word in the banner. I have a slightly simpler PR in #5282 that just puts almost everything inside of a function, so we don't have to worry about splitting the work up into pre-compile and post-compile.

[ivarne]

Yes, it is probably not that much overhead in doing everything on start up. It will anyway only get executed when using the REPL.

[staticfloat]

It's still getting precompiled, as well. It's just not getting precached.

[staticfloat]

I'd like to change that print() to a warn(), so that it's coming out of STDERR.

Other than that, I'd say this is good to merge.

[ivarne]

I think print -> warn is a different issue, unrelated to this.

After static compilation this error is not even printed when it triggers. When the git version PR gets merged, we already give a warning during compilation if git is unavailable, and I hope someone will notice if we put an invalid VERSION in the repository.

Also one would need change the include order in sysimg.jl because warn is not defined when version.jl is executed. If you think this is an important warning we must be clear on when it should be issued. Would println(STDERR, "while creating Base.VERSION, ignoring error $e") be good enough? (if it was not ignored)

[staticfloat]

Let's leave it for the git version PR then. It was more a matter of principle, and I figured that since we've change so much of this file we might as well change that as well. In general, I'd like our error and warning reporting mechanisms to be as consistent as possible, but it's not a blocking issue at all.

[staticfloat]

Can you merge, or should I do it?

[ivarne]

I agree with the consistency issue. Reporting warnings during compilation is different from runtime warnings and I don't know if we have/ should have a standard way of reporting those. Are there many of them?

I can't merge anything because I don't have commit access, so you have to pull the trigger for me.