enhance: respect git config merge.ff

[sina-hide]

This adds another configuration possibility for the default merge mode to the already existing per branch git configuration branch.<name>.mergeoptions (#540) and the per repo preference "Preferred Merge Mode" (#1156).

Defined values:

• false: do no fast-forward merge
• only: do only a fast-forward merge

This is configurable by for example git config set --global merge.ff <value> for a global (meaning the current user) configuration.

The priority between these configurations/preferences is:

• git configuration branch.<name>.mergeoptions ([feature] default settings for merge options should be taken from .gitconfig #540)
• git configuration merge.ff
• preference "Preferred Merge Mode" (Remember (or preset) the merge-flags option (in combo-box) #1156)

[sina-hide]

@love-linger In addition to adding this feature I have done a few refactorings regarding MergeMode. If you don't like them, I can revert them.

[love-linger]

I think the Preferred Merge Mode in repository configuration (which can be traited as local setting) should use higher-priority than git config merge.ff.

I'll merge this first and modify it myself.

[sina-hide]

I think the Preferred Merge Mode in repository configuration (which can be traited as local setting) should use higher-priority than git config merge.ff.

Okay!

I'll merge this first and modify it myself.

There was a little problem so merge.ff didn't work. See PR #1501.

[love-linger]

I just realized that when the user selects the Default merge mode, SourceGit does not pass parameters related to the merge mode to the git merge command. That is to say, if the user selects the Default merge mode, the merge.ff configuration should take effect automatically. We don't need the GetGitConfigMergeFF function.

Moreover, the description of the Default mode may be a bit misleading. It should not be Fast-forward if possible, but rather Use git configuration.

[sina-hide]

You are right, @love-linger. Your corresponding commit is d3d1377.