switch to os.UserHomeDir() to detect user home directory

[sersh88]

When install shell completion scripts it's more accurate to use os.UserHomeDir().
Program can be run with sudo and will use root's homeDir instead of original user.

[posener]

Looks good.
The problem is that this function was introduced in go1.12 (see failing builds).
I assume we need to split the implementation to pre go 1.12 and go 1.12+?

[codecov]

Codecov Report

Merging #119 into master will decrease coverage by 2.47%.
The diff coverage is 71.31%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   86.56%   84.08%   -2.48%     
==========================================
  Files           8        9       +1     
  Lines         454      509      +55     
==========================================
+ Hits          393      428      +35     
- Misses         45       62      +17     
- Partials       16       19       +3     

Impacted Files	Coverage Δ
flags.go	66.66% <0.00%> (ø)
internal/arg/arg.go	100.00% <ø> (ø)
predict/predict.go	100.00% <ø> (ø)
testing.go	42.85% <ø> (ø)
compflag/compflag.go	63.15% <61.97%> (-7.68%)	⬇️
predict/options.go	80.64% <80.64%> (ø)
complete.go	96.87% <100.00%> (+0.12%)	⬆️

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[posener]

🙂
This is not what I meant.
In Go you can tell the compiler which go to compile in which go version using build tags.
So you can create two files, one that will be called userdir_go1_11.go that will be compiled with go 1.11 and below, and another file userdir.go for newer versions.
This files will contain an internal function that returns the user directory, but one of them will contain the old code and the other will contain the new API. Does that make sense?