rhg: support "!" syntax for disabling extensions

This makes it so that calls in test-log.t do not fall back immediately because
of the disabled extension, instead going through the CLI parsing code, which
breaks because of invalid UTF-8 in a flag.

I *think* clap 3.x+ supports this? I'm not sure, and we have to upgrade the
minimum Rust version to use clap 3.x anyway which is out of scope for this
series, so let's just kick that can down the road a little bit.

rust/rhg/src/main.rs

@@ -687,13 +687,18 b' fn check_extensions(config: &Config) -> '
     }
 
     let enabled: HashSet<&[u8]> = config
-        .get_section_keys(b"extensions")
-        .into_iter()
-        .map(|extension| {
+        .iter_section(b"extensions")
+        .filter_map(|(extension, value)| {
+            if value == b"!" {
+                // Filter out disabled extensions
+                return None;
+            }
             // Ignore extension suboptions. Only `required` exists for now.
             // `rhg` either supports an extension or doesn't, so it doesn't
             // make sense to consider the loading of an extension.
-            extension.split_2(b':').unwrap_or((extension, b"")).0
+            let actual_extension =
+                extension.split_2(b':').unwrap_or((extension, b"")).0;
+            Some(actual_extension)
         })
         .collect();
 

tests/test-log.t

@@ -2157,6 +2157,8 b' test -u/-k for problematic encoding'
   ... '''.encode('utf-8')) and None
   $ sh < setup.sh
 
+#if no-rhg
+
 test in problematic encoding
   >>> with open('test.sh', 'wb') as f:
   ...     f.write(u'''
@@ -2179,6 +2181,8 b' test in problematic encoding'
   3
   1
 
+#endif
+
   $ cd ..
 
 test hg log on non-existent files and on directories