Bug in overlapping content when ignoring everything

[bep]

Discovered while adding test cases for #12912.

A little hard to explain, but this test setup did not work as expected:

-- hugo.toml --
theme = "t1"
[[module.mounts]]
source = 'content'
target = 'content'
[[module.mounts]]
source = 'content2'
target = 'content/c2'
[[module.mounts]]
source = 'content3'
target = 'content/watchdisabled'
disableWatch = true
[[module.mounts]]
source = 'content4'
target = 'content/excludedsome'
excludeFiles = 'p1.md'
[[module.mounts]]
source = 'content5'
target = 'content/excludedall'
excludeFiles = '/**'
[[module.mounts]]
source = "hugo_stats.json"
target = "assets/watching/hugo_stats.json"
-- hugo_stats.json --
Some stats.
-- content/foo.md --
foo
-- content2/bar.md --
-- themes/t1/layouts/_default/single.html --
{{ .Content }}
-- themes/t1/static/f1.txt --
-- content3/p1.md --
-- content4/p1.md --
-- content4/p2.md --
-- content5/p3.md --
-- content5/p4.md --

The excludeFiles = '/**' effectively excluded the entire mount (correct), but due to a logic error in the code, we skipped all the other content mounts mounted to the same target root.

I don't suspect this to be a common setup, though.

[github-actions]

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