log: fix --follow null parent not to include revision 0
If p1 is null, ':.' is translated as '0:null'. But rangeset can't handle null,
only revision 0 was visible. Because 'null' should not be listed implicitly,
"log --follow" (without -r) should be empty if p1 is null.
Test of "hg grep -f" is added for cmdutil.walkchangerevs().
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.