log --author/--committer: really match only with name part

log --author/--committer: really match only with name part

When we tried to find commits done by AUTHOR, the first implementation
tried to pattern match a line with "^author .*AUTHOR", which later was
enhanced to strip leading caret and look for "^author AUTHOR" when the
search pattern was anchored at the left end (i.e. --author="^AUTHOR").

This had a few problems:

 * When looking for fixed strings (e.g. "git log -F --author=x --grep=y"),
   the regexp internally used "^author .*x" would never match anything;

 * To match at the end (e.g. "git log --author='google.com>$'"), the
   generated regexp has to also match the trailing timestamp part the
   commit header lines have.  Also, in order to determine if the '$' at
   the end means "match at the end of the line" or just a literal dollar
   sign (probably backslash-quoted), we would need to parse the regexp
   ourselves.

An earlier alternative tried to make sure that a line matches "^author "
(to limit by field name) and the user supplied pattern at the same time.
While it solved the -F problem by introducing a special override for
matching the "^author ", it did not solve the trailing timestamp nor tail
match problem.  It also would have matched every commit if --author=author
was asked for, not because the author's email part had this string, but
because every commit header line that talks about the author begins with
that field name, regardleses of who wrote it.

Instead of piling more hacks on top of hacks, this rethinks the grep
machinery that is used to look for strings in the commit header, and makes
sure that (1) field name matches literally at the beginning of the line,
followed by a SP, and (2) the user supplied pattern is matched against the
remainder of the line, excluding the trailing timestamp data.

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