Actually switch to "Resolving Deltas" (#4080)

Libgit2's "Resolving Deltas" code is extremely slow (libgit2/libgit2#4674) on
larger repositories, so it is important to have an accurate progress bar to avoid users thinking
the download is stuck. We had this implemented. However, we were never actually switching to it,
because the progress meter thought the progress was jumping backwards and wouldn't actually update
because of it. Fix that by resetting it on the first switch to resolving deltas.

src/GitTools.jl

@@ -11,14 +11,18 @@ import LibGit2
 using Printf
 
 use_cli_git() = Base.get_bool_env("JULIA_PKG_USE_CLI_GIT", false)
+const RESOLVING_DELTAS_HEADER = "Resolving Deltas:"
 
 function transfer_progress(progress::Ptr{LibGit2.TransferProgress}, p::Any)
     progress = unsafe_load(progress)
     @assert haskey(p, :transfer_progress)
     bar = p[:transfer_progress]
     @assert typeof(bar) == MiniProgressBar
     if progress.total_deltas != 0
-        bar.header = "Resolving Deltas:"
+        if bar.header != RESOLVING_DELTAS_HEADER
+            bar.header = RESOLVING_DELTAS_HEADER
+            bar.prev = 0
+        end
         bar.max = progress.total_deltas
         bar.current = progress.indexed_deltas
     else