all: update for Git

- rewrite .gitignore to use git patterns
- mv hg(1) to git(1) and rewrite
- add lib/git/commit-msg.hook
- add skeleton codereview script
- update codereview(1)

Change-Id: I061cd8e4de77ebbd6037a7c5d1582cd1d986f62f

.gitignore

@@ -1,27 +1,23 @@
-(^|/)CVS($|/)
-(^|/)\.hg($|/)
-(^|/)\.hgtags($|/)
-^plan9.log$
-^plan9-reverse.log$
-^state$
-^state.old$
-^state.journal$
-\.o$
-\.a$
-^man/man[0-9]+/.*\.html$
-^src/.*/o\.[^/]+$
-y.tab.[ch]$
-^src/cmd/rc/x.tab.h$
-^unix/.*\.tgz$
-^bin/
-^log/
-^dict/
-^postscript/font/
-\.orig$
+plan9.log
+plan9-reverse.log
+state
+state.old
+state.journal
+*.o
+*.a
+man/man?/*.html
+src/**/o.*
+y.tab.[ch]
+src/cmd/rc/x.tab.h
+unix/*.tgz
+bin/
+log/
+dict/
+postscript/font/
+*.orig
 config
 install.log
 install.sum
 last-change
-lib/codereview/codereview.pyc
 src/cmd/awk/y.output
 src/cmd/devdraw/latin1.h