Let users help diff-mode find files to patch

[Jim Blandy]

When using Mercurial Queues, if some patch named
ROOT/.hg/patches/fix-blurgh.patch contains hunks for some file
a/urgh/blurgh.cpp, then those hunks always apply to
ROOT/urgh/blurgh.cpp.

However, Emacs's diff-mode isn't smart enough to figure that out,
meaning that commands like `diff-goto-source' or `diff-apply-hunk'
prompt for the filename in an annoying way.

The attached patch adds a -function to diff-mode that users can
customize to tell diff-mode how to do the right thing.

diff-find-file-name-functions.patch
Description: Text Data