Make restyle-diff pick a better base to restyle against

[ksperling-apple]

• Default to upstream/master if upstream exists
• Use --merge-base to restrict to files changed on current branch

[github-actions]

PR #25887: Size comparison from 2aefaa6 to ca3541e

Decreases (1 build for cc32xx)

platform	target	config	section	2aefaa6	ca3541e	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	20310044	20310043	-1	-0.0

Full report (1 build for cc32xx)

platform	target	config	section	2aefaa6	ca3541e	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	645769	645769	0	0.0
			(read/write)	203848	203848	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197248	197248	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	933102	933102	0	0.0
			.debug_aranges	87704	87704	0	0.0
			.debug_frame	301596	301596	0	0.0
			.debug_info	20310044	20310043	-1	-0.0
			.debug_line	2681024	2681024	0	0.0
			.debug_loc	2827360	2827360	0	0.0
			.debug_ranges	286352	286352	0	0.0
			.debug_str	3041144	3041144	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105953	105953	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	380543	380543	0	0.0
			.symtab	257472	257472	0	0.0
			.text	537696	537696	0	0.0