[Bug-vc-dwim] bug#38862: attribution line parsing

[Karl Berry]

Hi Jim - When I had only a single space between date and name in the
attribution line, as in:

2020-01-01 Karl Berry  <address@hidden>

vcd gave no errors by default. Then I accidentally ran vcd --commit with
that CL and the only error (warning) was at the end, direct from Perl:

Use of uninitialized value $author in sprintf at /u/karl/gnu/src/vkarl/VC.pm 
line 210.
Use of uninitialized value $author in sprintf at /u/karl/gnu/src/vkarl/VC.pm 
line 210.

I was not left in a good state.  There seemed to be no good way to
recover from this, since the erroneous ChangeLog was already committed,
so vcd did not think there were any files in the ChangeLog lines.

So I ended up doing rm -rf .git/c/.git and running vcd --init again to
restart my CL repo, and also
  git fetch && git reset --hard origin && git checkout && git submodule update
to eradicate all my changes (after saving the actual diff :) in the
source repo. Thankfully, that apparently erased the memory of the bad CL
so I could proceed.

Instead of worrying about the exact number of spaces, I wonder about
parsing attr lines by checking that first word is a YYYY-MM-DD date, the
last word is <address@hidden>, and considering the words in between to be
the real name.

Reasonable error msgs would then be easy to give if the first and last
words don't fit the expected syntax.  And if there is no author, it
could give up at that point. After all, vcd relies on all kinds of
finicky details about the CL, so seems like it might as well force a
good attr line too instead of accepting bad ones.

If the idea seems ok, maybe one of us (or a new contributor!) can work
in that direction eventually. --best, karl.