Run include order check in doc-check

Now the doc-check target also checks that all files conform to the
include ordering produced by the include sorter.

Add support into the include sorter for only checking the ordering
without changing anything, and partially improve things such that the
full contents of the file are no longer required for some parts of the
checking.  There seems to be no performance impact for now from storing
all the file contents in memory, so did not go through this, but the
partial changes also improve readability of the code.
Add support to gmxtree for loading the git attributes, to know which
files not to check for include ordering.

Change-Id: I919850dab2dfa742f9fb5b216cc163bc118082cc