Allow non-developer to clone, checkout and fetch more easily.

Allow non-developer to clone, checkout and fetch more easily.

The code that uses committer_info() in reflog can barf and die
whenever it is asked to update a ref.  And I do not think
calling ignore_missing_committer_name() upfront like recent
receive-pack did in the aplication is a reasonable workaround.

What the patch does.

 - git_committer_info() takes one parameter.  It used to be "if
   this is true, then die() if the name is not available due to
   bad GECOS, otherwise issue a warning once but leave the name
   empty".  The reason was because we wanted to prevent bad
   commits from being made by git-commit-tree (and its
   callers).  The value 0 is only used by "git var -l".

   Now it takes -1, 0 or 1.  When set to -1, it does not
   complain but uses the pw->pw_name when name is not
   available.  Existing 0 and 1 values mean the same thing as
   they used to mean before.  0 means issue warnings and leave
   it empty, 1 means barf and die.

 - ignore_missing_committer_name() and its existing caller
   (receive-pack, to set the reflog) have been removed.

 - git-format-patch, to come up with the phoney message ID when
   asked to thread, now passes -1 to git_committer_info().  This
   codepath uses only the e-mail part, ignoring the name.  It
   used to barf and die.  The other call in the same program
   when asked to add signed-off-by line based on committer
   identity still passes 1 to make sure it barfs instead of
   adding a bogus s-o-b line.

 - log_ref_write in refs.c, to come up with the name to record
   who initiated the ref update in the reflog, passes -1.  It
   used to barf and die.

The last change means that git-update-ref, git-branch, and
commit walker backends can now be used in a repository with
reflog by somebody who does not have the user identity required
to make a commit.  They all used to barf and die.

I've run tests and all of them seem to pass, and also tried "git
clone" as a user whose GECOS is empty -- git clone works again
now (it was broken when reflog was enabled by default).

But this definitely needs extra sets of eyeballs.

Signed-off-by: Junio C Hamano <junkio@cox.net>