Git: gh-pages branch diverged from master

Is someone making changes to the gh-pages branch? I tried to rebase from master to have it pick up some DPUB-AAM changes and got a number of conflicts from a "Snapshots 4 Dec 2014" commit. I would have expected a simple fast-forward would work.

Is there a legitimate reason for the divergence, or was this a mistake? May I --force it to match master?

Thanks,
James


$ git co gh-pages
Branch gh-pages set up to track remote branch gh-pages from origin.
Switched to a new branch 'gh-pages'

$ git rebase master
First, rewinding head to replay your work on top of it...
Applying: Snapshots 4 Dec 2014
Using index info to reconstruct a base tree...
M	accname-aam/accname-aam.html
M	aria/aria.html
M	core-aam/core-aam.html
<stdin>:112: trailing whitespace.
    text-transform: uppercase;   
<stdin>:127: trailing whitespace.
    background: #fcfaee;    
<stdin>:277: trailing whitespace.
.permalink a, .permalink a:link, .permalink a:visited, .permalink a:hover, .permalink a:focus, .permalink a:active 
<stdin>:309: trailing whitespace.
em.rfc2119 { 
<stdin>:421: trailing whitespace.
      
warning: squelched 967 whitespace errors
warning: 972 lines add whitespace errors.
Falling back to patching base and 3-way merge...
Auto-merging core-aam/core-aam.html
CONFLICT (content): Merge conflict in core-aam/core-aam.html
Auto-merging aria/aria.html
CONFLICT (content): Merge conflict in aria/aria.html
Auto-merging accname-aam/accname-aam.html
CONFLICT (content): Merge conflict in accname-aam/accname-aam.html
Failed to merge in the changes.
Patch failed at 0001 Snapshots 4 Dec 2014
The copy of the patch that failed is found in:
   ~/gitroot/aria/.git/rebase-apply/patch

When you have resolved this problem, run "git rebase --continue".
If you prefer to skip this patch, run "git rebase --skip" instead.
To check out the original branch and stop rebasing, run "git rebase --abort".

$ git st
# rebase in progress; onto 257f65f
# You are currently rebasing branch 'gh-pages' on '257f65f'.
#   (fix conflicts and then run "git rebase --continue")
#   (use "git rebase --skip" to skip this patch)
#   (use "git rebase --abort" to check out the original branch)
#
# Unmerged paths:
#   (use "git reset HEAD <file>..." to unstage)
#   (use "git add <file>..." to mark resolution)
#
#	both modified:      accname-aam/accname-aam.html
#	both modified:      aria/aria.html
#	both modified:      core-aam/core-aam.html
#
no changes added to commit (use "git add" and/or "git commit -a")