fix: validate branch in bench update

bench update assumes that erpnext is an installed app in the bench. This is a wrong assumption. This work around checks for the intersection of installed apps with  the list ['frappe', 'erpnext'] and only attempts to get the current branch of apps that occur in the intersection. This way, if erpnext is not installed (very likely for those using frappe only to build their own apps) there wont be an error when get_current_branch is called for erpnext. The need to perform the kind of check being performed in validate_branch is a strong case for considering apps being able to add "bench update" hooks in their hooks.py

See this topic https://discuss.erpnext.com/t/bench-update-error-no-such-file-or-directory-apps-erpnext/56932?u=chude_osiegbu

bench/app.py

@@ -423,11 +423,14 @@ def get_apps_json(path):
 		return json.load(f)
 
 def validate_branch():
-	for app in ['frappe', 'erpnext']:
+	installed_apps = get_apps()
+	check_apps = ['frappe', 'erpnext'] #this is a dirty work-around. bench update should not assume that erpnext is installed
+	intersection_apps = list(set(installed_apps) & set(check_apps))
+	for app in intersection_apps:
 		branch = get_current_branch(app)
 
 		if branch == "master":
 			print(''' master branch is renamed to version-11 and develop to version-12. Please switch to new branches to get future updates.
 
 To switch to version 11, run the following commands: bench switch-to-branch version-11''')
-			sys.exit(1)
+			sys.exit(1)