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

templates/repo/commits_table.tmpl

@@ -18,7 +18,7 @@
 						<input type="checkbox" name="all" id="all" value="true" {{.All}}>
 						<label for="all">{{.i18n.Tr "repo.commits.search_all"}} &nbsp;&nbsp;</label>
 					</div>
-					<button class="ui black tiny button" data-panel="#add-deploy-key-panel">{{.i18n.Tr "repo.commits.find"}}</button>
+					<button class="ui black tiny button" data-panel="#add-deploy-key-panel" data-tooltip={{.i18n.Tr "repo.commits.search.tooltip"}}>{{.i18n.Tr "repo.commits.find"}}</button>
 				</form>
 			{{else if .IsDiffCompare}}
 				<a href="{{$.CommitRepoLink}}/commit/{{.BeforeCommitID}}" class="ui green sha label">{{ShortSha .BeforeCommitID}}</a> ... <a href="{{$.CommitRepoLink}}/commit/{{.AfterCommitID}}" class="ui green sha label">{{ShortSha .AfterCommitID}}</a>