[commit] Add setting for merge message format

[aik099]

Currently the log message format for merge commits is hardcoded and looks like this:

Merging from Trunk to Stable
* r1111: PRJ-1234 - Task Description
1. change1 made in this task
2. change1 made in this task
* r2222: PRJ-5555 - Task Description
1. change1 made in this task
2. change1 made in this task
* r3333: PRJ-1234 - Task Description
1. change1 made in this task
2. change1 made in this task

This is by revision printing. In #7 a suggestion is made to change that format to another one, which by bug printing.

Instead I propose to add commit.merge-message-format setting, that would allow to select from list of supported formats. To make this happen the config command needs to be changed to allow selection from fixed list of options instead of opening interactive editor for this new setting.

Possible formats:

• auto
• by revision
• by task
• summary only

By default auto format would be selected, that will split merged revisions list into 2 groups:

• ones that have task
• ones that don't have task

Then corresponding commit message would be generated for each group and combined into final message.

[aik099]

The 24dea2c commit created by task format (merge template called group_by_bug), that will put all merged revisions without detected bug ids into Revisions without Bug IDs: group which effectively eliminates need for auto mode.