Make svn-create-patch's diffs of ObjC header files more readable

Fixes <http://webkit.org/b/36055>.

Reviewed by John Sullivan.

* Scripts/svn-create-patch:
(diffOptionsForFile): Added. Returns the options that should be passed
to diff for the given file. All the options are the same for all
files, except for the option to specify which lines should be used as
hunk headers.
(generateDiff): Use diffOptionsForFile to get the options to pass to
diff.
(hunkHeaderLineRegExForFile): Added. Returns the regular expression
that should be used by diff to identify lines that should be included
after the "@@" in the hunk header lines of the diff. For ObjC[++]
source files, we use any lines starting with -, +, or
@implementation/@interface/@protocol. For ObjC[++] header files (which
we assume to be any .h files in a mac/ or objc/ directory), we use any
lines starting with @implementation/@interface/@protocol.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@55898 268f45cc-cd09-0410-ab3c-d52691b4dbfc