Better ignoring with new -m option and improvements to -C option.

[chrsmj]

• The -m option maps to diff -I option to ignore lines matching a regex pattern. The GUI still lists different files matching this pattern in the main window, but the separate viewer window will only show the line if it is with the nearby -c option specified context range of some other difference. Potentially useful if the files you are comparing are only different by one fairly inconsequential line, such as a uuid produced by another process.
• The -C option was extended to ignore files specified in both .gitignore files as well as the previous .cvsignore files. This option was also refined to carry on elegantly when either file is missing and just ignore the defaults provided for elsewhere in the file, including a new .git/ default ignore to skip those directories wherever they may be in the tree.