Bootstrap: Make file that saves the last boot branch to be hidden

That file doesn't need to be visible for it to work

.gitignore

@@ -26,7 +26,7 @@ spyder_crash.log
 .cache
 .coverage*
 MANIFEST
-boot_branch.txt
+.boot_branch.txt
 
 # git .orig files
 *.orig

bootstrap.py

@@ -81,7 +81,7 @@ assert args.gui in (None, 'pyqt5', 'pyside2'), \
 installed_dev_repo = False
 if not args.no_install:
     prev_branch = None
-    boot_branch_file = DEVPATH / "boot_branch.txt"
+    boot_branch_file = DEVPATH / ".boot_branch.txt"
     if boot_branch_file.exists():
         prev_branch = boot_branch_file.read_text()