README: update help message and specify git requirement

README.md

@@ -14,7 +14,8 @@ my everyday use.
 Download the latest [release][releases] from GitHub.  Binaries are available for
 Linux and macOS.
 
-You must have a moderately recent copy of git in your PATH to run gg.
+You must have a moderately recent copy of git in your PATH to run gg. gg is
+tested against 2.7.4 and newer. Older versions may work, but are not supported.
 
 [releases]: https://github.com/zombiezen/gg/releases
 
@@ -47,11 +48,14 @@ basic commands:
   clone         make a copy of an existing repository
   commit        commit the specified files or all outstanding changes
   diff          diff repository (or selected files)
+  histedit      interactively edit revision history
   init          create a new repository in the given directory
   log           show revision history of entire repository or files
   mail          creates or updates a Gerrit change
+  merge         merge another revision into working directory
   pull          pull changes from the specified source
   push          push changes to the specified destination
+  rebase        move revision (and descendants) to a different branch
   remove        remove the specified files on the next commit
   revert        restore files to their checkout state
   status        show changed files in the working directory
@@ -62,6 +66,8 @@ options:
     	path to git executable
   -show-git
     	log git invocations
+  -version
+    	display version information
 ```
 
 ## License