give input option to continue or not

Summary:
to ask for input option:

1) If not running in master branch, ask for continue or not.
The default option is "n".

2) If local version is different with "origin/master",
to ask for doing 'git pull' or not. The default option
is "y".

Test Plan:
Test in my local mac when switch in different branch
and revision.

Reviewers: benkraft, csilvers

Reviewed By: benkraft, csilvers

Differential Revision: https://phabricator.khanacademy.org/D55648

git_sync.sh

@@ -1,25 +1,33 @@
 #!/usr/bin/env bash
 
+# Load shared setup functions.
+SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd -P )"
+. "$SCRIPT_DIR/shared-functions.sh"
+
 # Find the git repo, branch, etc in "khan-dotfiles"
 REPO="$(basename `git rev-parse --show-toplevel`)"
 BRANCH="$(git rev-parse --abbrev-ref HEAD 2>/dev/null)"
 LOCAL_SHA=$(git rev-parse --verify HEAD)
-REMOTE_SHA=$(git rev-parse --verify FETCH_HEAD)
+REMOTE_SHA=$(git ls-remote . | grep refs/remotes/origin/HEAD | cut -f 1)
 
+echo "Welcome to Khan-dotfiles!"
 if [ -z "$BRANCH" ]; then
     echo "Error: Could not locate a git branch in $REPO"
     exit 1
 fi
 
 # Check if we're on the master branch.
 if [ "$BRANCH" != "master" ]; then
-    echo "Your directory is running branch \"$BRANCH\""
-    echo "You need to run in master branch at \"$REPO\""
-    exit 1
-fi
+    echo "Running branch \"$BRANCH\", not master."
+    [ ! "$(get_yn_input "Would you like to continue?" "n")" != "y" ]
+    exit $?
 
+fi
 if [ $LOCAL_SHA != $REMOTE_SHA ]; then
-    echo "your local repo \"$REPO\" is different with 'origin/master'"
-    echo "please 'git pull' and re-run 'make' "
-    exit 1
+    echo "your local repo \"$REPO\" is different from 'origin/master'"
+    if [ "$(get_yn_input "Would you like to run 'git pull'?" "y")" = "y" ]; then
+        git pull
+    else
+        exit 0
+    fi
 fi