Some refactors about GetMergeBase

[lunny]

Maybe fix #32018

• Use gitrepo.GetMergeBase method instead of other two implementations.
• Add FetchRemoteCommit so that we don't need to add many remote to the git repository to avoid possible git lock conflicts. A lock will start when invoke the function, it will be invoked when cross-repository comparing. The head repository will fetch the base repository's base commit id. In most situations, it should lock the fork repositories so that it should not become a bottleneck.
• Improve GetCompareInfo to remove unnecessarily adding remote.
• Remove unnecessary parameters of SignMerge.

[wxiaoguang]

CI failure is also related.

Do rerun the CI multiple times before merge to ensure there is no regression.

[wxiaoguang]

CI failure is also related.

Do rerun the CI multiple times before merge to ensure there is no regression.

CI failure is still related

     pull_create_test.go:98: 
        	Error Trace:	/home/runner/work/gitea/gitea/tests/integration/integration_test.go:348
        	            				/home/runner/work/gitea/gitea/tests/integration/integration_test.go:160
        	            				/home/runner/work/gitea/gitea/tests/integration/pull_create_test.go:98
        	            				/home/runner/work/gitea/gitea/tests/integration/pull_create_test.go:399
        	            				/opt/hostedtoolcache/go/1.25.5/x64/src/sync/waitgroup.go:239
        	            				/opt/hostedtoolcache/go/1.25.5/x64/src/runtime/asm_amd64.s:1693
        	Error:      	Not equal: 
        	            	expected: 200
        	            	actual  : 500
        	Test:       	TestPullCreateParallel
        	Messages:   	Request: GET /user2/repo1/compare/master...user1/repo1:new-branch-3


    pull_create_test.go:416: 
        	Error Trace:	/home/runner/work/gitea/gitea/tests/integration/pull_create_test.go:416
        	            				/home/runner/work/gitea/gitea/tests/integration/git_helper_for_declarative_test.go:88
        	            				/home/runner/work/gitea/gitea/tests/integration/pull_create_test.go:384
        	Error:      	Not equal: 
        	            	expected: 8
        	            	actual  : 7
        	Test:       	TestPullCreateParallel
    pull_create_test.go:417: 
        	Error Trace:	/home/runner/work/gitea/gitea/tests/integration/pull_create_test.go:417
        	            				/home/runner/work/gitea/gitea/tests/integration/git_helper_for_declarative_test.go:88
        	            				/home/runner/work/gitea/gitea/tests/integration/pull_create_test.go:384
        	Error:      	Not equal: 
        	            	expected: 8
        	            	actual  : 7
        	Test:       	TestPullCreateParallel

[lunny]

CI failure is also related.

Do rerun the CI multiple times before merge to ensure there is no regression.

Added a lock for the fetch operations to the repository and rerun the CI many times and all PASS.