Some Git merge drivers can partially resolve conflicts, leaving some
conflict markers in the output file. In that case, they exit with a code
between 1 and 127 to indicate that there was a conflict remaining in the
file (since Git uses a shell to run the merge drivers, and shells often
use exit codes above 127 for special meanings such as exiting due to a
signal):
https://git-scm.com/docs/gitattributes#_defining_a_custom_merge_driver
We should support this convention as well, since it will allow many Git
merge drivers to be used with Jujutsu, but we don't run our merge tools
through a shell, so there is no reason to treat exit codes 1 through 127
specially. Instead, we let the user specify which exact exit codes
should indicate conflicts. This is also better for cross-platform
support, since Windows may use different exit codes than Linux for
instance.