git_command.py: Handle unicode decode error

repo diffmanifests saves git commit messages in buf and uses default
utf-8 decoding, in some scenarios git commit message can itself contain
a non UTF-8 character due to a typo or incorrect i18n.commitEncoding.

e.g.
d354d9afe923 [PATCH] fbcon: don\xb4t call set_par() in fbcon_init() if vc_mode == KD_GRAPHICS

Convert the buf containing git commits to string if decoding to utf-8
encounters an error.

Signed-off-by: Gaurav Pathak <gaurav.pathak@pantacor.com>
Change-Id: If818562f0faaa5062c765fbea11dc0e1c86a24d7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/294742


Reviewed-by: Mike Frysinger <vapier@google.com>