log --author: take union of multiple "author" requests

In the olden days,

    log --author=me --committer=him --grep=this --grep=that

used to be turned into:

    (OR (HEADER-AUTHOR me)
        (HEADER-COMMITTER him)
        (PATTERN this)
        (PATTERN that))

showing my patches that do not have any "this" nor "that", which was
totally useless.

80235ba ("log --author=me --grep=it" should find intersection, not union,
2010-01-17) improved it greatly to turn the same into:

    (ALL-MATCH
      (HEADER-AUTHOR me)
      (HEADER-COMMITTER him)
      (OR (PATTERN this) (PATTERN that)))

That is, "show only patches by me and committed by him, that have either
this or that", which is a lot more natural thing to ask.

We however need to be a bit more clever when the user asks more than one
"author" (or "committer"); because a commit has only one author (and one
committer), they ought to be interpreted as asking for union to be useful.
The current implementation simply added another author/committer pattern
at the same top-level for ALL-MATCH to insist on matching all, finding
nothing.

Turn

    log --author=me --author=her \
    	--committer=him --committer=you \
	--grep=this --grep=that

into

    (ALL-MATCH
      (OR (HEADER-AUTHOR me) (HEADER-AUTHOR her))
      (OR (HEADER-COMMITTER him) (HEADER-COMMITTER you))
      (OR (PATTERN this) (PATTERN that)))

instead.

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

grep.h

@@ -22,6 +22,7 @@ enum grep_header_field {
 	GREP_HEADER_AUTHOR = 0,
 	GREP_HEADER_COMMITTER
 };
+#define GREP_HEADER_FIELD_MAX (GREP_HEADER_COMMITTER + 1)
 
 struct grep_pat {
 	struct grep_pat *next;
@@ -41,6 +42,7 @@ enum grep_expr_node {
 	GREP_NODE_ATOM,
 	GREP_NODE_NOT,
 	GREP_NODE_AND,
+	GREP_NODE_TRUE,
 	GREP_NODE_OR
 };