split_ident_line(): make best effort when parsing author/committer line

split_ident_line(): make best effort when parsing author/committer line

Commits made by ancient version of Git allowed committer without
human readable name, like this (00213b17c in the kernel history):

    tree 6947dba41f8b0e7fe7bccd41a4840d6de6a27079
    parent 352dd1df32e672be4cff71132eb9c06a257872fe
    author Petr Baudis <pasky@ucw.cz> 1135223044 +0100
    committer  <sam@mars.ravnborg.org> 1136151043 +0100

    kconfig: Remove support for lxdialog --checklist

    ...

    Signed-off-by: Petr Baudis <pasky@suse.cz>
    Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
When fed such a commit, --format='%ci' fails to parse it, and gives
back an empty string.  Update the split_ident_line() to be a bit
more lenient when parsing, but make sure the caller that wants to
pick up sane value from its return value does its own validation.

Signed-off-by: Junio C Hamano <gitster@pobox.com>