If there are too many files in .git/refs/heads/, git packs them in
.git/packed-refs.  This breaks the build as gen/angle/angle_commit.h
depends on .git/refs/heads/<branch-name>.

With this change, when gn is run and the dependency to this file is set
up, it's unpacked from .git/packed-refs if it's missing from
.git/refs/heads/.

One corner case not solved by this change is if a commit is amended with
no changes and git packs the refs (and so the
.git/refs/heads/<branch-name> file goes away), ninja doesn't rerun gn as
it doesn't believe there to be any changes, yet fails the build as the
dependency is now missing.  Running `gn gen` manually fixes the issue.
The issue is automatically fixed when switching branches.

Bug: angleproject:6131
Change-Id: Ie2404e874a5cb3374a652b93faa143acb68eaa19
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3001906Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>