get_maintainer: allow keywords to match filenames

Allow K: entries in MAINTAINERS to match directly against filenames;
either those extracted from patch +++ or --- lines, or those specified
on the command-line using the -f option.

This potentially allows fewer lines in a MAINTAINERS entry, if all the
relevant files are scattered throughout the whole kernel tree, yet
contain some common keyword.  An example would be using an ARM SoC name
as the keyword to catch all related drivers.

I don't think setting exact_pattern_match_hash would be appropriate
here; at least for intended Tegra use case, this feature is to ensure
that all Tegra-related driver changes get Cc'd to the Tegra mailing
list.  Setting exact_pattern_match_hash would prevent git history
parsing for e.g.  S-o-b tags, which still seems like it would be useful.
Hence, this flag isn't set.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>