am: reload .gitattributes after patching it

am: reload .gitattributes after patching it

When applying multiple patches with git am, or when rebasing using the
am backend, it's possible that one of our patches has updated a
gitattributes file. Currently, we cache this information, so if a
file in a subsequent patch has attributes applied, the file will be
written out with the attributes in place as of the time we started the
rebase or am operation, not with the attributes applied by the previous
patch. This problem does not occur when using the -m or -i flags to
rebase.

To ensure we write the correct data into the working tree, expire the
cache after each patch that touches a path ending in ".gitattributes".
Since we load these attributes in multiple separate files, we must
expire them accordingly.

Verify that both the am and rebase code paths work correctly, including
the conflict marker size with am -3.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>