Change on check-attr behavior

[Sérgio Peixoto]

Hi,

I think there is a bug on the check-attr behavior of git when asking
for the "allowed-ext" attribute.  Check the logs below to see that
with version  2.20.1.windows.1 we get unspecified even the attribute
is there as you can see when asking for all the attributes.

=== OLD VERSION ===
> git version
git version 2.8.1.windows.1

> git check-attr -a test.py
test.py: text: set
test.py: allowed-ext: 100

> git check-attr allowed-ext test.py
test.py: allowed-ext: 100

=== NEW VERSION ===
> git --version
git version 2.20.1.windows.1

> git check-attr -a test.py
test.py: text: set
test.py: allowed-ext: 100

> git check-attr allowed-ext test.py
test.py: allowed-ext: unspecified

Cheers,
Sérgio Peixoto