ui: add configoverride context manager

I feel like this idea might've been discussed before, so please
feel free to point me to the right mailing list entry to read
about why it should not be done.

We have a common pattern of the following code:
    backup = ui.backupconfig(section, name)
    try:
        ui.setconfig(section, name, temporaryvalue, source)
        do_something()
    finally:
        ui.restoreconfig(backup)

IMO, this looks better:
    with ui.configoverride({(section, name): temporaryvalue}, source):
        do_something()

Especially this becomes more convenient when one has to backup multiple
config values before doing something. In such case, adding a new value
to backup requires codemod in three places.

mercurial/ui.py

@@ -7,6 +7,7 b''
 
 from __future__ import absolute_import
 
+import contextlib
 import errno
 import getpass
 import inspect
@@ -1193,6 +1194,23 b' class ui(object):'
                 " update your code.)") % version
         self.develwarn(msg, stacklevel=2, config='deprec-warn')
 
+    @contextlib.contextmanager
+    def configoverride(self, overrides, source=""):
+        """Context manager for temporary config overrides
+        `overrides` must be a dict of the following structure:
+        {(section, name) : value}"""
+        backups = {}
+        for (section, name), value in overrides.items():
+            backups[(section, name)] = self.backupconfig(section, name)
+            self.setconfig(section, name, value, source)
+        yield
+        for __, backup in backups.items():
+            self.restoreconfig(backup)
+        # just restoring ui.quiet config to the previous value is not enough
+        # as it does not update ui.quiet class member
+        if ('ui', 'quiet') in overrides:
+            self.fixconfig(section='ui')
+
 class paths(dict):
     """Represents a collection of paths and their configs.