plain: add a diff exception

Summary:
Oculus wants to preserve the "do not show binary diff" config while running
`hg export` in automation, since printing the binary diff might just OOM the
container. Let's add a plain exception for it.

Reviewed By: zhh95

Differential Revision: D13111212

fbshipit-source-id: 34af58ac0917de3b3231e637774896d882585e26

mercurial/patch.py

@@ -2365,7 +2365,7 @@ def difffeatureopts(
             # True, or False.
             if v or isinstance(v, bool):
                 return v
-        if forceplain is not None and ui.plain():
+        if forceplain is not None and ui.plain("diffopts"):
             return forceplain
         return getter(section, name or key, untrusted=untrusted)
 
@@ -2377,7 +2377,7 @@ def difffeatureopts(
     }
     buildopts["worddiff"] = (
         ui.configbool("experimental", "worddiff")
-        and not ui.plain()
+        and not ui.plain("diffopts")
         and ui._colormode is not None
     )
 

tests/test-diff-binary.t

@@ -0,0 +1,23 @@
+  $ newrepo
+  $ setconfig diff.git=1 diff.nobinary=1
+
+  >>> open('a.bin', 'wb').write(b'\0\1')
+  $ hg commit -m A -A a.bin
+
+  >>> open('a.bin', 'wb').write(b'\0\2')
+
+  $ hg diff
+  diff --git a/a.bin b/a.bin
+  Binary file a.bin has changed
+
+  $ HGPLAIN=1 hg diff
+  diff --git a/a.bin b/a.bin
+  index bdc955b7b2e610ad5a72302b139a2e6cb325519a..8835708590a9afa236e1bbad18df9d23de82ccd3
+  GIT binary patch
+  literal 2
+  Jc${Nk0ssI600RI3
+  
+
+  $ HGPLAIN=1 HGPLAINEXCEPT=diffopts hg diff
+  diff --git a/a.bin b/a.bin
+  Binary file a.bin has changed