help: only call doc() when it is callable
`hg help -k` on my machine was aborting because the hg-prompt extension
was inserting a string and not a function into help.helptable and help
was blindly calling it.
This patch changes keyword searching to be more robust against
unexpected types.
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing:
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See http://mercurial.selenic.com/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.