Support search operators for commits search (#6479)

* Support searching commits with prefix syntax

For now, support auther: committer:
When more than one prefix is supplied is presented, the result is the union.
When different prefixes are supplied, the result is the intersection.

For example,
"author:alice author:bob"
=> the result is all commits authored by Alice OR Bob

"hello committer:alice"
=> the result is all commits committed by Alice AND has the keyword
'hello' in the message.

Note that there should NOT have any space after the colon(:) of the prefix.
For example,
"author:bill" => correct
"author: bill" => wrong

* Remove unneeded logging

* Add missing files of test repository

* Add missing repo_unit entries to test fixtures

* Update test cases

* Add tooltip for commits search button

* Update tooltip text

I have no idea about how to format it with line breaks.

* Make the usage example more real

* Add a test case

* Add new options struct for SearchCommits

* Prefer len(s) > 0 over s != ""

* Add NewSearchCommitsOptions

integrations/api_repo_test.go

@@ -70,9 +70,9 @@ func TestAPISearchRepo(t *testing.T) {
 		expectedResults
 	}{
 		{name: "RepositoriesMax50", requestURL: "/api/v1/repos/search?limit=50", expectedResults: expectedResults{
-			nil:   {count: 19},
-			user:  {count: 19},
-			user2: {count: 19}},
+			nil:   {count: 20},
+			user:  {count: 20},
+			user2: {count: 20}},
 		},
 		{name: "RepositoriesMax10", requestURL: "/api/v1/repos/search?limit=10", expectedResults: expectedResults{
 			nil:   {count: 10},