feat(log): add option to search for header or body to git log

Note to reviewer: I hate the name `--header-or`!  Please help me come up
with a better name.

Summary:
This change adds a new option to `git log` that allows users to search
for commits that match either the author or the commit message. This is
useful for finding commits that were either authored or co-authored by a
specific person.

Currently, the best way to find a commit either authored or co-authored
by a specific person is to use

```sh
echo \
    $(git log --author=Torvalds --pretty="%cd,%H\n" --date=iso-strict) \
    $(git log --grep="Co-authored-by: .*Torvalds" --pretty="%cd,%H\n" --date=iso-strict) \
| sort -n --reverse \
| awk -F, '{print $2}' \
| tr '\n' '\t' \
| xargs git show --stat --stdin
```

This is a bit of a pain, so this change adds a new option to `git log`.
Now finding either authors or co-authors is as simple as

```sh
git log --author=Torvalds --grep=Torvalds --header-or
```

Test plan:
1. create commit authored by A and co-authored-by B
2. create commit authored by B
3. run `git log --author=B --grep="Co-authored-by: B" --header-or`
4. expect to see both commits

Signed-off-by: Max 👨🏽‍💻 Coplan <[email protected]>

grep.c

@@ -797,7 +797,7 @@ void compile_grep_patterns(struct grep_opt *opt)
 	if (opt->no_body_match && opt->pattern_expression)
 		opt->pattern_expression = grep_not_expr(opt->pattern_expression);
 
-	if (!header_expr)
+	if (!header_expr || opt->header_or)
 		return;
 
 	if (!opt->pattern_expression)

grep.h

@@ -147,6 +147,7 @@ struct grep_opt {
 	int count;
 	int word_regexp;
 	int all_match;
+	int header_or;
 	int no_body_match;
 	int body_hit;
 #define GREP_BINARY_DEFAULT	0

revision.c

@@ -2646,6 +2646,9 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->grep_filter.pattern_type_option = GREP_PATTERN_TYPE_PCRE;
 	} else if (!strcmp(arg, "--all-match")) {
 		revs->grep_filter.all_match = 1;
+	// @@@ must-fix: find a better name
+	} else if (!strcmp(arg, "--header-or")) {
+		revs->grep_filter.header_or = 1;
 	} else if (!strcmp(arg, "--invert-grep")) {
 		revs->grep_filter.no_body_match = 1;
 	} else if ((argcount = parse_long_opt("encoding", argv, &optarg))) {

t/t7810-grep.sh

@@ -961,6 +961,17 @@ test_expect_success 'log --grep --author uses intersection' '
 	test_cmp expect actual
 '
 
+test_expect_success 'log --grep --author --header-or uses union' '
+	# grep matches only third and fourth
+	# author matches only initial and third
+	git log --author="A U Thor" --grep=r --header-or --format=%s >actual &&
+	{
+	    echo fourth && echo third
+	} >expect &&
+	test_cmp expect actual
+'
+
+
 test_expect_success 'log --grep --grep --author takes union of greps and intersects with author' '
 	# grep matches initial and second but not third
 	# author matches only initial and third