Pkg.jl: Print diff when a tracked branch changes

[fredrikekre]

I think it would be useful to print a diff when a tracked branch is updated. Current behaviour:

pkg> add BenchmarkTools#master

# change the master branch

pkg> up
[...]
  Updating `~/.julia/environments/v0.7/Project.toml`
 [6e4b80f9] ~ BenchmarkTools v0.2.5+ #master
  Updating `~/.julia/environments/v0.7/Manifest.toml`
 [6e4b80f9] ~  BenchmarkTools v0.2.5+ #master

which IMO would be better as

pkg> up
[...]
  Updating `~/.julia/environments/v0.7/Project.toml`
 [6e4b80f9] ↑ BenchmarkTools v0.2.5+ #master(commit1) ⇒ v0.2.5+ #master(commit2)
  Updating `~/.julia/environments/v0.7/Manifest.toml`
 [6e4b80f9] ↑ BenchmarkTools v0.2.5+ #master(commit1) ⇒ v0.2.5+ #master(commit2)

Additionally, perhaps we should always print the commit hash for tracked branches, e.g. st would change from

(v0.7) pkg> st
Status `~/.julia/environments/v0.7/Project.toml`
 [6e4b80f9] BenchmarkTools v0.2.5+ #master

to

(v0.7) pkg> st
Status `~/.julia/environments/v0.7/Project.toml`
 [6e4b80f9] BenchmarkTools v0.2.5+ #master(commit)

[KristofferC]

We actually don't store the commit SHA (only the tree sha) for tracked branches. We could though.