git: don't redundantly pass "--" to git_sha1
git_sha1 already puts "--" before its arguments, so
git_sha1_file($dir, 'doc/index.mdwn')
would have incorrectly invoked
git rev-list --max-count=1 HEAD -- -- doc/index.mdwn
If there is no file in the wiki named "--", that's harmless, because
it merely names the latest revision in which either "--" or
"doc/index.mdwn" changed. However, it could return incorrect results
if there is somehow a file named "--".
(cherry picked from commit
0463357392471f58ccd2c7f39cafabd8472e129b)