[Bug] mtree_mutate incorrectly strips root directory prefixes

[j-eid]

It looks like #852 causes issues with strip_prefix in mtree_mutate when stripping the root directory.

bazel-lib/lib/private/modify_mtree.awk

Lines 17 to 20 in 250c835

	components = split($1, _, "/");
	if ($0 ~ /type=dir/ && components == 1) {
	$1 = $1 "/";
	}

This causes a / to be added to the first keyword in the mtree specification.

Steps to reproduce

# BUILD

genrule(
    name = "test.mf",
    outs = ["test.mf.spec"],
    cmd = "printf 'root_dir/ uid=0 gid=0 time=0 mode=0755 type=dir\nroot_dir/child_dir/ uid=0 gid=0 time=0 mode=0755 type=dir' >$@",
)

mtree_mutate(
    name = "test_mutate.mf",
    mtree = ":test.mf",
    strip_prefix = "root_dir",
)

Result:

# test_mutate.mf.spec

uid=0/ gid=0 time=0 mode=0755 type=dir
child_dir/ uid=0 gid=0 time=0 mode=0755 type=dir

The entry should probably be deleted unless package_dir is provided. I'm not very familiar with awk syntax, but another check for whether package_dir is provided and a more complex check on L18 would probably fix this.