suppress git_protocol message in git_sitrep (r-lib#2039)

* suppress git_protocol message in git_sitrep

`git_protocol()` sets a default *and* messages
the user if the option isn't already set.
This message looks confusing in the output for
`git_sitrep()`. This commit suppresses that
messsage.

* replace suppressMessages() with ui_silence()

Author: thomascwells

R/git.R

@@ -357,7 +357,7 @@ git_sitrep <- function(tool = c("git", "github"),
     if (!vaccinated) {
       ui_bullets(c("i" = "See {.fun usethis::git_vaccinate} to learn more."))
     }
-    kv_line("Default Git protocol", git_protocol())
+    kv_line("Default Git protocol", ui_silence(git_protocol()))
     kv_line("Default initial branch name", init_default_branch)
   }