config: Add .git/.revupconfig and reorder precedence

[jerry-skydio]

Adds support for a per-checkout config at .git/.revupconfig with the
highest file precedence: .git/.revupconfig > repo/.revupconfig >
~/.revupconfig.

This allows users to specify custom overrides without needing to either
commit or ignore the changes. It also allows us to swap ~/.revupconfig
to now taking the lowest precedence. The original reason it was above
repo config was because we wanted a way for the user to override the
config provided by the repo. However now the config inside.git provides
that, and having ~/ be lowest precedence is more similar to the
functionality of other dotfiles (like gitconfig).

[jerry-skydio]

Reviews in this chain:
└#260 config: Add .git/.revupconfig and reorder precedence

[jerry-skydio]

#	head	base	diff	date	summary
0	38e8d358	3417332b	diff	Jun 9 16:05 PM	5 files changed, 99 insertions(+), 11 deletions(-)
1	75dc9efa	3417332b	diff	Jun 9 16:23 PM	4 files changed, 50 insertions(+), 3 deletions(-)
2	76454ad6	3417332b	diff	Jun 9 17:09 PM	1 file changed, 3 insertions(+), 9 deletions(-)
3	e176551c	aab90897	rebase	Jun 19 12:49 PM	0 files changed
4	242888f8	aab90897	diff	Jun 19 13:02 PM	4 files changed, 79 insertions(+), 12 deletions(-)
5	63719dac	aab90897	diff	Jun 19 13:07 PM	1 file changed, 2 insertions(+), 2 deletions(-)