Format git log to include only commit shas

I am running into an issue during import where a commit message would contain the "^M" control character. This will cause codeq to error on a StringIndexOutOfBoundsException.
```
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 40
	at java.lang.String.substring(String.java:1907)
	at clojure.core$subs.invoke(core.clj:4517)
	at datomic.codeq.core$commits$fn__69$fn__70.invoke(core.clj:428)
	at clojure.core$mapv$fn__6258.invoke(core.clj:6241)
```

Formatting the git log "pretty" option to return only commit shas will eliminate the need to parse commit messages in bulk, and reduce destructuring in the argument list of some functions.