bootstrap: remove the need for a sorted .gitignore

bootstrap: remove the need for a sorted .gitignore

During bootstrap, files may be created which are already included
in .gitignore, but the test to add such a file relied on the
sort order.  Now, it just adds such a new entry and thus only
changes the file if the line count would change.

* bootstrap (insert_if_absent): Instead of comparing the
sorted new file with the original, the function compares the line
count, and only in case of a difference, the given file is changed.
Also ensure that the given ignore file does not already include
duplicate entries, as otherwise, the entry count would be innacurate.
(sort_patterns): Remove this now redundant function.
(gitignore_entries): A new function to return significant entries
from .gitignore.

Improved-by: Pádraig Brady