Update stderr wording of git_not_command

This changed in git v2.13.1, see
https://github.com/gitster/git/commit/6c486862636be1fe2d5785451c52f5379b0bad24#diff-081cf476dd9ac3b05c183570de47cb23

tests/rules/test_git_not_command.py

@@ -7,7 +7,7 @@ from tests.utils import Command
 def git_not_command():
     return """git: 'brnch' is not a git command. See 'git --help'.
 
-Did you mean this?
+The most similar command is
 branch
 """
 
@@ -16,7 +16,7 @@ branch
 def git_not_command_one_of_this():
     return """git: 'st' is not a git command. See 'git --help'.
 
-Did you mean one of these?
+The most similar commands are
 status
 reset
 stage
@@ -29,7 +29,7 @@ stats
 def git_not_command_closest():
     return '''git: 'tags' is not a git command. See 'git --help'.
 
-Did you mean one of these?
+The most similar commands are
 \tstage
 \ttag
 '''