Single revision selection
During development, many times you want to select a single revision in the history of a project, to examine it, or to compare with the current version. The ability to a select revision is also the basis for selecting a revision range, for example a subsection of history to examine.
Many Git commands take revision parameters as arguments, which is typically denoted by <rev> in Git reference documentation. Git allows you to specify specific commits or a range of commits in several ways.
HEAD – the implicit revision
Most, but not all, Git commands that require the revision parameter, default to using HEAD. For example, git log and git log HEAD will show the same information.
The HEAD denotes the current branch, or in other words the commit that was checked out into the working directory, and forms a base of a current work.
There are a few other references which are similar to HEAD:
FETCH_HEAD: This records the information about the remote branches that were fetched...
