Flags and parameters to be parsed.
+A revision parameter typically names a commit object. They use
+what is called an 'extended SHA1' syntax.
+* The full SHA1 object name (40-byte hexadecimal string), or
+ a substring of such that is unique within the repository.
+ E.g. dae86e1950b1277e545cee180551750029cfe735 and dae86e both
+ name the same commit object if there are no other object in
+ your repository whose object name starts with dae86e.
+* A symbolic ref name. E.g. 'master' typically means the commit
+ object referenced by $GIT_DIR/refs/heads/master. If you
+ happen to have both heads/master and tags/master, you can
+ explicitly say 'heads/master' to tell GIT which one you mean.
+* A suffix '^' to a revision parameter means the first parent of
+ that commit object. '^<n>' means the <n>th parent (i.e. 'rev^'
+ is equivalent to 'rev^1'). As a special rule,
+ 'rev^0' means the commit itself and is used when 'rev' is the
+ object name of a tag object that refers to a commit object.
+* A suffix '~<n>' to a revision parameter means the commit
+ object that is the <n>th generation grand-parent of the named
+ commit object, following only the first parent. I.e. rev~3 is
+ equivalent to rev^^^ which is equivalent to rev^1^1^1.
+'git-rev-parse' also accepts a prefix '^' to revision parameter,
+which is passed to 'git-rev-list'. Two revision parameters
+concatenated with '..' is a short-hand for writing a range
+between them. I.e. 'r1..r2' is equivalent to saying '^r1 r2'
Written by Linus Torvalds <email@example.com> and