Add 'git history' alias

git log -p <filename> will show you the commit history that affects a
particular file (or the current branch, or another ref) and shows the
patch / diff of changes.

I find it pretty useful, even if git history is more to type, it's
easier for me to remember, and tab completion is fantastic.

git-aliases

@@ -7,3 +7,4 @@
   today = log --since "yesterday"
   squash = "!_git_squash() { git rebase -i HEAD~$1; }; _git_squash"
   stashed = stash list --pretty=format:'%gd: %h [%ar] %s'
+  history = log -p