tools: Make remote configurable in {reset,push}-to-pull-request.

The appropriate name for the remote pointing at the repo we maintain
may be `upstream` for most of our repos... but not when we're
downstream of someone else, e.g. for react-native.  So, make it easy
to configure per-repo.

tools/push-to-pull-request

@@ -13,13 +13,18 @@ GitHub to see the subsequent push to master as representing a
 merge of the PR, rather than requiring the PR to be manually
 (and to the casual observer misleadingly) closed instead.
 
+REMOTE defaults to the value of the Git config variable
+\`zulip.zulipRemote\` if set, else to \`upstream\`.
+
 See also \`reset-to-pull-request\`.
 EOF
     exit 1
 }
 
+remote_default="$(git config zulip.zulipRemote || echo upstream)"
+
 pr_id="$1"
-remote="${2:-upstream}"
+remote="${2:-"$remote_default"}"
 
 if [ -z "$pr_id" ]; then
     usage

tools/reset-to-pull-request

@@ -1,6 +1,5 @@
 #!/bin/bash
 set -e
-set -x
 
 if ! git diff-index --quiet HEAD; then
     set +x
@@ -9,7 +8,12 @@ if ! git diff-index --quiet HEAD; then
     echo "Doing nothing to avoid losing your work."
     exit 1
 fi
+
+remote_default="$(git config zulip.zulipRemote || echo upstream)"
+
 request_id="$1"
-remote=${2:-"upstream"}
+remote=${2:-"$remote_default"}
+
+set -x
 git fetch "$remote" "pull/$request_id/head"
 git reset --hard FETCH_HEAD