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

models/fixtures/repo_unit.yml

@@ -220,4 +220,39 @@
   repo_id: 28
   type: 1
   config: "{}"
-  created_unix: 1524304355
+  created_unix: 1524304355
+
+-
+  id: 33
+  repo_id: 36
+  type: 4
+  config: "{}"
+  created_unix: 1524304355
+
+-
+  id: 34
+  repo_id: 36
+  type: 5
+  config: "{}"
+  created_unix: 1524304355
+
+-
+  id: 35
+  repo_id: 36
+  type: 1
+  config: "{}"
+  created_unix: 1524304355
+
+-
+  id: 36
+  repo_id: 36
+  type: 2
+  config: "{\"EnableTimetracker\":true,\"AllowOnlyContributorsToTrackTime\":true}"
+  created_unix: 1524304355
+
+-
+  id: 37
+  repo_id: 36
+  type: 3
+  config: "{\"IgnoreWhitespaceConflicts\":false,\"AllowMerge\":true,\"AllowRebase\":true,\"AllowRebaseMerge\":true,\"AllowSquash\":true}"
+  created_unix: 1524304355

models/fixtures/repository.yml

@@ -430,3 +430,14 @@
   num_forks: 0
   num_issues: 0
   is_mirror: false
+
+-
+  id: 36
+  owner_id: 2
+  lower_name: commits_search_test
+  name: commits_search_test
+  is_private: false
+  num_stars: 0
+  num_forks: 0
+  num_issues: 0
+  is_mirror: false

models/fixtures/user.yml

@@ -28,7 +28,7 @@
   is_admin: false
   avatar: avatar2
   avatar_email: user2@example.com
-  num_repos: 6
+  num_repos: 7
   num_stars: 2
   num_followers: 2
   num_following: 1