[release-branch.go1.5] build: Fix bootstrap.bash for official source tarballs

At the moment, bootstrap.bash assumes it is called from a git working
copy. Hence, it fails to complete when running in an unpacked official
source tarball where .git and .gitignore do not exist. This fix adds a
test for existence for .git and a -f switch for the removal of
.gitignore.

Fixes #12223

Change-Id: I7f305b83b38d5115504932bd38dadb7bdeb5d487
Reviewed-on: https://go-review.googlesource.com/13770


Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-on: https://go-review.googlesource.com/14281