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/gitea-repositories-meta/user2/commits_search_test.git/objects/0a/8499a22ad32a80beda9d75efe15f9f94582468

@@ -0,0 +1,2 @@
+x
<01><><EFBFBD>
+<EFBFBD>0<10>=<3D>)<29>.<2E>&<26><>-<2D><>;<3B>q<><71>BcK<63><4B><EFBFBD>Ļ<><C4BB>a<EFBFBD>F<>Z<EFBFBD>i<>6U<36>!)<29><>d<EFBFBD><14><>=$<24><>'<27><>cr<63><72><EFBFBD>(4<>i<EFBFBD>g<03>(<28>D<EFBFBD><44>2<EFBFBD>l3f<33><66>1<EFBFBD><31><EFBFBD><1A>C%ҫ=<3D>

integrations/gitea-repositories-meta/user2/commits_search_test.git/objects/6e/8eabd9a7f8d6acd2a1219facfd37415564b144

@@ -0,0 +1,2 @@
+x
<01><>[
+1E<><45>*<2A>/J2<4A>+ <20>K<EFBFBD>dL<>\<5C>E܀_.<2E>{<7B>պv<D5BA> <20><><EFBFBD>
[!<21>J<EFBFBD>1&<26><><EFBFBD>I<EFBFBD>=e$-<2D>q6eI<65><49><EFBFBD><EFBFBD><EFBFBD><EFBFBD>p<EFBFBD>V5<56>|q<>w<EFBFBD><77><EFBFBD><EFBFBD><EFBFBD><EFBFBD>u\<06>ā'8bFtc<1D>nI?<3F>D<EFBFBD><03><>1<EFBFBD>

integrations/gitea-repositories-meta/user2/commits_search_test.git/objects/c2/0caf78b5f9dd2d0d183876c5cd0e761c13f7f8

@@ -0,0 +1,2 @@
+x
<01><>K
+1]<5D>ً<><D98B>o<EFBFBD>0<EFBFBD>x<EFBFBD>Nҍ<4E>1<EFBFBD><10>㛅p<>E<15><>[{