[Prefix] Make get_addable_spec start search from HEAD of upstream repo (#219)

* [Prefix] Make `get_addable_spec` start search from HEAD of upstream repo

Co-authored-by: Elliot Saba <staticfloat@gmail.com>

* Update src/Prefix.jl

Co-authored-by: Elliot Saba <staticfloat@gmail.com>

Co-authored-by: Elliot Saba <staticfloat@gmail.com>

Author: giordano

src/Prefix.jl

@@ -477,7 +477,14 @@ function get_addable_spec(name::AbstractString, version::VersionNumber;
 
         LibGit2.with(LibGit2.GitRepo(dir)) do repo
             LibGit2.with(LibGit2.GitRevWalker(repo)) do walker
-                LibGit2.push_head!(walker)
+                # The repo is cached, so locally it may be checking out an outdated commit.
+                # Start the search from HEAD of the tracking upstream repo.
+                try
+                    LibGit2.push!(walker, LibGit2.GitHash(LibGit2.peel(LibGit2.GitCommit, LibGit2.upstream(LibGit2.head(repo)))))
+                catch
+                    @warn("Could not walk from origin branch!")
+                    LibGit2.push_head!(walker)
+                end
                 # For each commit in the git repo, check to see if its treehash
                 # matches the one we're looking for.
                 for oid in walker