Fix scoop not using proxy for git commands (#843)

Commands that call git from scoop will now pass proxy details. The
command is executed using `cmd` to avoid polluting the user's
environment as the variables set will die with the process.

Fixes #842

lib/git.ps1

@@ -0,0 +1,25 @@
+function git_proxy_cmd {
+    $proxy = $(scoop config proxy)
+    $cmd = "SET HTTPS_PROXY=$proxy&&SET HTTP_PROXY=$proxy&&git $($args |% { "$_ " })"
+    cmd /c $cmd
+}
+
+function git_clone {
+    git_proxy_cmd clone $args
+}
+
+function git_ls_remote {
+    git_proxy_cmd ls-remote $args
+}
+
+function git_pull {
+    git_proxy_cmd pull $args
+}
+
+function git_fetch {
+    git_proxy_cmd fetch $args
+}
+
+function git_log {
+    git_proxy_cmd log $args
+}