GitTimeParser: Fix multiple errorprone and style comments

This code came from GitDateParser and inherited these issues. It is a
good time to fix them:

* Use always {} in if and fors (eclipse guide)

* Use the more efficient EnumMap for the formatters

* variable.equals(CONSTANT) instead of CONSTANT.equals(var)
  https://errorprone.info/bugpattern/YodaCondition

* private constructor to prevent instantiations of a utility classes
  https://errorprone.info/bugpattern/PrivateConstructorForUtilityClass

* methods named in camelCase

* Use string.split(..., int)
  https://errorprone.info/bugpattern/StringSplitter

* Initialize variable in first use

Change-Id: I0ae70ad9befc66fa9c057af135f2b0b2dab3869a