fix(scm): add onDidChangeCommitTemplate event

[zWingz]

Signed-off-by: zwingz [email protected]

What it does

add onDidChangeCommitTemplate event to SCM extensions, close #9778

In vscode-git, when our repo has some change, It will get the inputTemplate and set it to SCM input (updateModalState)
and it will call getInputTemplate and getMergeMessage

The SCM will call updateSourceControl and fire onDidChangeCommitTemplateEmitter

But Theia does not provide a onDidChangeCommitTemplate listeners

So the vscode-git cann't update the SCM-input
Vscode References

How to test

• remove example/browser dependencies @theia/git
• add [email protected] to theiaPlugins
• start example browser
• merge two conflicting branches

mkdir test-merge-conflict
cd test-merge-conflict
echo 'master' > a.txt
git add . && git commit -m 'from master'
git checkout -b conflict-1
echo 'conflict-1' > a.txt
git add . && git commit -m 'from conflict-1'
git checkout master
echo 'master modified` > a.txt
git add . && git commit -m 'from master 2`
git merge conflit

Note the SCM input, should be with value Merge branch 'conflict'

[msujew]

I can confirm that the issue exists on master and is addressed nicely by this change. Very clean solution 👍

State after trying to merge on master:

State after trying to merge with the PR: