Show HEAD as NeogitBranchHead in Log Graph

In the log graph it's non intuitive where the currently checked out
branch is. To make this more customizable for the user, we add the
`NeogitBranchHead` highlight group.

doc/neogit.txt

@@ -200,6 +200,7 @@ exist, they will be created with sensible defaults based on your colorscheme.
 
 STATUS BUFFER
 NeogitBranch                Header showing currently checked out branch
+NeogitBranchHead            Currently checkoued branch (in log graph)
 NeogitRemote                Header showing current branch's remote (if set)
 NeogitObjectId              Object's SHA hash
 NeogitStash                 Stash name

lua/neogit/buffers/common.lua

@@ -120,24 +120,27 @@ M.CommitEntry = Component.new(function(commit, args)
       remote_name, local_name = local_name, remote_name
     end
 
+    local is_head = string.match(commit.ref_name, "HEAD") ~= nil
+    local branch_highlight = is_head and "NeogitBranchHead" or "NeogitBranch"
+
     if local_name and remote_name and vim.endswith(remote_name, local_name) then
       local remote = remote_name:match("^([^/]*)/.*$")
       table.insert(ref, text(remote .. "/", { highlight = "NeogitRemote" }))
-      table.insert(ref, text(local_name, { highlight = "NeogitBranch" }))
+      table.insert(ref, text(local_name, { highlight = branch_highlight }))
       table.insert(ref, text(" "))
     else
       if local_name then
         table.insert(
           ref,
-          text(local_name, { highlight = local_name:match("/") and "NeogitRemote" or "NeogitBranch" })
+          text(local_name, { highlight = local_name:match("/") and "NeogitRemote" or branch_highlight })
         )
         table.insert(ref, text(" "))
       end
 
       if remote_name then
         table.insert(
           ref,
-          text(remote_name, { highlight = remote_name:match("/") and "NeogitRemote" or "NeogitBranch" })
+          text(remote_name, { highlight = remote_name:match("/") and "NeogitRemote" or branch_highlight })
         )
         table.insert(ref, text(" "))
       end

lua/neogit/lib/hl.lua

@@ -164,6 +164,7 @@ function M.setup()
     NeogitCommandCodeNormal = { link = "String" },
     NeogitCommandCodeError = { link = "Error" },
     NeogitBranch = { fg = palette.blue, bold = true },
+    NeogitBranchHead = { fg = palette.blue, bold = true, underline = true },
     NeogitRemote = { fg = palette.green, bold = true },
     NeogitUnmergedInto = { fg = palette.bg_purple, bold = true },
     NeogitUnpushedTo = { fg = palette.bg_purple, bold = true },