debugshell: allow commands to be specified as a CLI argument

Add a `--command` option to `hg debugshell` that allows the user to pass in
Python code to evaluate directly from the command line. This was inspired by
the `--command` option present in Facebook's Sapling fork of Mercurial,
which in turn was inspired by the `-c` option of the Python interpreter
itself. It is particularly useful for writing tests, especially for getting
visibility into things that otherwise aren't exposed via debug commands.

mercurial/debugcommands.py

@@ -3786,8 +3786,21 b' def debugsub(ui, repo, rev=None):'
         ui.writenoi18n(b' revision %s\n' % v[1])
 
 
-@command(b'debugshell', optionalrepo=True)
-def debugshell(ui, repo):
+@command(
+    b'debugshell',
+    [
+        (
+            b'c',
+            b'command',
+            b'',
+            _(b'program passed in as a string'),
+            _(b'COMMAND'),
+        )
+    ],
+    _(b'[-c COMMAND]'),
+    optionalrepo=True,
+)
+def debugshell(ui, repo, **opts):
     """run an interactive Python interpreter
 
     The local namespace is provided with a reference to the ui and
@@ -3815,6 +3828,12 b' def debugshell(ui, repo):'
         except ImportError:
             site = None  # Keep PyCharm happy
 
+    command = opts.get('command')
+    if command:
+        compiled = code.compile_command(encoding.strfromlocal(command))
+        code.InteractiveInterpreter(locals=imported_objects).runcode(compiled)
+        return
+
     code.interact(local=imported_objects)
 
 

tests/test-completion.t

@@ -328,7 +328,7 b' Show all commands + options'
   debugrevspec: optimize, show-revs, show-set, show-stage, no-optimized, verify-optimized
   debugserve: sshstdio, logiofd, logiofile
   debugsetparents: 
-  debugshell: 
+  debugshell: command
   debugsidedata: changelog, manifest, dir
   debugssl: 
   debugstrip: rev, force, no-backup, nobackup, , keep, bookmark, soft

tests/test-debugcommands.t

@@ -724,3 +724,8 b' Test debugpeer'
   pushable: yes
 
 #endif
+
+Test debugshell
+
+  $ hg debugshell -c 'ui.write(b"%s\n" % ui.username())'
+  test