merge: backport GIT_MERGE_AUTOEDIT support

merge: backport GIT_MERGE_AUTOEDIT support

Even though 1.7.9.x series does not open the editor by default
when merging in general, it does do so in one occassion: when
merging an annotated tag. And worse yet, there is no good way
for older scripts to decline this.

Backport the support for GIT_MERGE_AUTOEDIT environment variable
from 1.7.10 track to help those stuck on 1.7.9.x maintenance
track.

Signed-off-by: Junio C Hamano <redacted>