patch: let caller pass in root-filtering matcher (API)

The --root option to `hg diff` does two things:

 * Shows paths relative to the given root

 * Filters paths by the given root, including copy sources

The root argument is passed through down to patch.diff(). I feel like
we can make patch.diff() more generic by not passing down the root
argument, but instead pass:

 * A function for taking a repo-relative path and printing it. I want
   to reuse this for showing cwd-relative paths later. This is the
   actual motivation for this patch.

 * A matcher that's already been filtered by the root argument

 * A second matcher that filters the copy sources

This is one step towards that.

Differential Revision: https://phab.mercurial-scm.org/D5892

mercurial/logcmdutil.py

@@ -74,6 +74,9 b' def diffordiffstat(ui, repo, diffopts, n'
                 ui.warn(_('warning: %s not inside relative root %s\n') % (
                     match.uipath(matchroot), uirelroot))
 
+        relrootmatch = scmutil.match(ctx2, pats=[relroot], default='path')
+        match = matchmod.intersectmatchers(match, relrootmatch)
+
     if stat:
         diffopts = diffopts.copy(context=0, noprefix=False)
         width = 80

mercurial/patch.py

@@ -32,7 +32,6 b' from . import ('
     encoding,
     error,
     mail,
-    match as matchmod,
     mdiff,
     pathutil,
     pycompat,
@@ -2320,10 +2319,6 b' def diffhunks(repo, ctx1, ctx2, match=No'
         return getfilectx
     getfilectx = lrugetfilectx()
 
-    if relroot:
-        relrootmatch = scmutil.match(ctx2, pats=[relroot], default='path')
-        match = matchmod.intersectmatchers(match, relrootmatch)
-
     if not changes:
         changes = ctx1.status(ctx2, match=match)
     modified, added, removed = changes[:3]