verify: move manifest verification to its own function
This makes verify more modular, making it easier for extensions to extend.
#require test-repo execbit
$ cd "`dirname "$TESTDIR"`"
look for python scripts without the execute bit
$ hg files 'set:**.py and not exec() and grep(r"^#!.*?python")'
[1]
look for python scripts with execute bit but not shebang
$ hg files 'set:**.py and exec() and not grep(r"^#!.*?python")'
[1]
look for shell scripts with execute bit but not shebang
$ hg files 'set:**.sh and exec() and not grep(r"^#!.*(ba)?sh")'
[1]
look for non scripts with no shebang
$ hg files 'set:exec() and not **.sh and not **.py and not grep(r"^#!")'
[1]