More verbose Pkg.update() for checked out packages

Just like how "free" packages display which versions they're updating from and to, display the git SHA for checked out packages as they are updated.  Here's a real-life example:

```txt
julia> Pkg.update()
INFO: Updating METADATA...
INFO: Updating Interpolations master...
INFO: Updating Tuples master...
INFO: Skipping TerminalExtensions (dirty)...
INFO: Updating AxisArrays master...
INFO: Updating Compat master...
INFO: Updating IntSets master...
INFO: Updating HDF5 master...
INFO: Skipping BitSets (dirty)...
INFO: Updating DataArrays master... b5d39c3 => c588781
INFO: Skipping Signals (dirty)...
INFO: Updating KDTrees master...
INFO: Updating FileIO master...
INFO: Skipping SmallDicts (dirty)...
INFO: Updating BinDeps master...
INFO: Updating DataFrames master...
INFO: Updating SIUnits master...
INFO: Updating Homebrew master...
INFO: Computing changes...
INFO: No packages to install, update or remove
```

To be honest, I was totally unaware of how many packages I had that were either dirty or happened to be on a custom branch before I made this change (DataArrays is only updating because I had been on a custom branch until I started printing out the branch names).  This also prints out the before and after SHA if there was a change.

base/pkg/entry.jl

@@ -290,11 +290,19 @@ function update(branch::AbstractString)
     for (pkg,ver) in fixed
         ispath(pkg,".git") || continue
         begin
-            if Git.attached(dir=pkg) && !Git.dirty(dir=pkg)
-                info("Updating $pkg...")
-                @recover begin
-                    Git.run(`fetch -q --all`, dir=pkg)
-                    Git.success(`pull -q --ff-only`, dir=pkg) # suppress output
+            if Git.attached(dir=pkg)
+                if Git.dirty(dir=pkg)
+                    info("Skipping $pkg (dirty)...")
+                else
+                    prev_sha = chomp(Git.readall(`rev-parse --short HEAD`, dir=pkg))
+                    @recover begin
+                        Git.run(`fetch -q --all`, dir=pkg)
+                        Git.success(`pull -q --ff-only`, dir=pkg) # suppress output
+                    end
+                    post_sha = chomp(Git.readall(`rev-parse --short HEAD`, dir=pkg))
+                    branch = chomp(Git.readall(`rev-parse --abbrev-ref HEAD`, dir=pkg))
+                    info("Updating $pkg $branch...", 
+                        prev_sha != post_sha ? " $prev_sha => $post_sha" : "")
                 end
             end
             if haskey(avail,pkg)