tests: Solaris diff -U also emits "No differences encountered"

This came up before, but the tests in check-code.py don't find -U (only -u)
and they don't work when the diff is inside a shell function.  This fixes
the offending tests and beefs up check-code.py.

contrib/check-code.py

@@ -122,7 +122,8 b' testpats = ['
     (r'^( *)\t', "don't use tabs to indent"),
     (r'sed (-e )?\'(\d+|/[^/]*/)i(?!\\\n)',
      "put a backslash-escaped newline after sed 'i' command"),
-    (r'^diff *-\w*u.*$\n(^  \$ |^$)', "prefix diff -u with cmp"),
+    (r'^diff *-\w*[uU].*$\n(^  \$ |^$)', "prefix diff -u/-U with cmp"),
+    (r'^\s+(if)? diff *-\w*[uU]', "prefix diff -u/-U with cmp"),
     (r'seq ', "don't use 'seq', use $TESTDIR/seq.py"),
     (r'\butil\.Abort\b', "directly use error.Abort"),
     (r'\|&', "don't use |&, use 2>&1"),

tests/test-merge-changedelete.t

@@ -460,7 +460,7 b' Force prompts with no input (should be s'
   --- file3 ---
   3
   changed2
-  $ diff -U8 $TESTTMP/fail.status $TESTTMP/prompt.status
+  $ cmp $TESTTMP/fail.status $TESTTMP/prompt.status || diff -U8 $TESTTMP/fail.status $TESTTMP/prompt.status
 
 
 Force prompts
@@ -583,7 +583,7 b' invocations.)'
   >         hg resolve --tool ":$tool" --all --config ui.interactive=True
   >         status > "$TESTTMP/compare.status" 2>&1
   >         echo '--- diff of status ---'
-  >         if diff -U8 "$TESTTMP/$tool.status" "$TESTTMP/compare.status"; then
+  >         if cmp "$TESTTMP/$tool.status" "$TESTTMP/compare.status" || diff -U8 "$TESTTMP/$tool.status" "$TESTTMP/compare.status"; then
   >             echo '(status identical)'
   >         fi
   >         lasttool="$tool"
@@ -868,7 +868,7 b' Force prompts with no input'
   --- file3 ---
   3
   changed1
-  $ diff -U8 $TESTTMP/fail.status $TESTTMP/prompt.status
+  $ cmp $TESTTMP/fail.status $TESTTMP/prompt.status || diff -U8 $TESTTMP/fail.status $TESTTMP/prompt.status
 
 Choose to merge all files
 

tests/test-merge-force.t

@@ -779,4 +779,4 b' Re-resolve and check status'
   warning: conflicts while merging missing_content2_missing_content4-untracked! (edit, then use 'hg resolve --mark')
   [1]
   $ checkstatus > $TESTTMP/status2 2>&1
-  $ diff -U8 $TESTTMP/status1 $TESTTMP/status2
+  $ cmp $TESTTMP/status1 $TESTTMP/status2 || diff -U8 $TESTTMP/status1 $TESTTMP/status2