use a default "fetch" strategy

[amtoine]

up until now, the default strategy for gm repo branch fetch has been none, which implies that, to have a proper "fetch strategy", --strategy <strat> had to be used every time...

however, in my Git config, i have pull.rebase set to true, which means i want to rebase most of the time.

this PR makes it so that the default value of strategy: string is either rebase when pull.rebase is set or merge when it's not.

the "strategy" can always be

• overwritten with --strategy rebase or --strategy merge
• disabled with --strategy none