doctor: suggest to install git if the system one is outdated

Closes Homebrew/homebrew#42934.

Signed-off-by: Baptiste Fontaine <batifon@yahoo.fr>

Library/Homebrew/cmd/doctor.rb

@@ -862,10 +862,13 @@ class Checks
     # https://help.github.com/articles/https-cloning-errors
     `git --version`.chomp =~ /git version ((?:\d+\.?)+)/
 
-    if $1 && Version.new($1) < Version.new("1.7.10") then <<-EOS.undent
+    if $1 && Version.new($1) < Version.new("1.7.10") then
+      git_upgrade_cmd = Formula["git"].any_version_installed? ? "upgrade" : "install"
+
+      <<-EOS.undent
         An outdated version of Git was detected in your PATH.
         Git 1.7.10 or newer is required to perform checkouts over HTTPS from GitHub.
-    Please upgrade: brew upgrade git
+        Please upgrade: brew #{git_upgrade_cmd} git
       EOS
     end
   end