Add ui.patch option.

ui.patch overrides the default patch/gpatch command and options.

doc/hgrc.5.txt

@@ -432,6 +432,9 b' ui::'
   merge;;
     The conflict resolution program to use during a manual merge.
     Default is "hgmerge".
+  patch;;
+    command to use to apply patches. Look for 'gpatch' or 'patch' in PATH if
+    unset.
   quiet;;
     Reduce the amount of output printed.  True or False.  Default is False.
   remotecmd;;

mercurial/commands.py

@@ -881,6 +881,8 b' def debuginstall(ui):'
     # patch
     ui.status(_("Checking patch...\n"))
     path = os.environ.get('PATH', '')
+    patcher = ui.config('ui', 'patch')
+    if not patcher:
     patcher = util.find_in_path('gpatch', path,
                                 util.find_in_path('patch', path, None))
     if not patcher:

mercurial/patch.py

@@ -293,9 +293,11 b' def patch(patchname, ui, strip=1, cwd=No'
         """patch and updates the files and fuzz variables"""
         fuzz = False
 
+        args = []
+        patcher = ui.config('ui', 'patch')
+        if not patcher:
         patcher = util.find_in_path('gpatch', os.environ.get('PATH', ''),
                                     'patch')
-        args = []
         if util.needbinarypatch():
             args.append('--binary')