Given three arguments, stores the <newvalue> in the <ref>, possibly dereferencing the symbolic refs, after verifying that the current value of the <ref> matches <oldvalue>. E.g. git update-ref
   refs/heads/master <newvalue> <oldvalue> updates the master branch head to <newvalue> only if its current value is <oldvalue>. You can specify 40 "0" or an empty string as <oldvalue> to make sure
   that the ref you are creating does not exist.

   It also allows a "ref" file to be a symbolic pointer to another ref file by starting with the four-byte header sequence of "ref:".

   More importantly, it allows the update of a ref file to follow these symbolic pointers, whether they are symlinks or these "regular file symbolic refs". It follows real symlinks only if they start
   with "refs/": otherwise it will just try to read them and update them as a regular file (i.e. it will allow the filesystem to follow them, but will overwrite such a symlink to somewhere else with a
   regular filename).

   If --no-deref is given, <ref> itself is overwritten, rather than the result of following the symbolic pointers.

   In general, using

       git update-ref HEAD "$head"

   should be a lot safer than doing

       echo "$head" > "$GIT_DIR/HEAD"

   both from a symlink following standpoint and an error checking standpoint. The "refs/" rule for symlinks means that symlinks that point to "outside" the tree are safe: they’ll be followed for
   reading but not for writing (so we’ll never write through a ref symlink to some other tree, if you have copied a whole archive by creating a symlink tree).

   With -d flag, it deletes the named <ref> after verifying it still contains <oldvalue>.

   With --stdin, update-ref reads instructions from standard input and performs all modifications together. Specify commands of the form:

       update SP <ref> SP <newvalue> [SP <oldvalue>] LF
       create SP <ref> SP <newvalue> LF
       delete SP <ref> [SP <oldvalue>] LF
       verify SP <ref> [SP <oldvalue>] LF
       option SP <opt> LF

   With --create-reflog, update-ref will create a reflog for each ref even if one would not ordinarily be created.

   Quote fields containing whitespace as if they were strings in C source code; i.e., surrounded by double-quotes and with backslash escapes. Use 40 "0" characters or the empty string to specify a zero
   value. To specify a missing value, omit the value and its preceding SP entirely.

   Alternatively, use -z to specify in NUL-terminated format, without quoting:

       update SP <ref> NUL <newvalue> NUL [<oldvalue>] NUL
       create SP <ref> NUL <newvalue> NUL
       delete SP <ref> NUL [<oldvalue>] NUL
       verify SP <ref> NUL [<oldvalue>] NUL
       option SP <opt> NUL

   In this format, use 40 "0" to specify a zero value, and use the empty string to specify a missing value.

   In either format, values can be specified in any form that Git recognizes as an object name. Commands in any other format or a repeated <ref> produce an error. Command meanings are:

   update
       Set <ref> to <newvalue> after verifying <oldvalue>, if given. Specify a zero <newvalue> to ensure the ref does not exist after the update and/or a zero <oldvalue> to make sure the ref does not
       exist before the update.

   create
       Create <ref> with <newvalue> after verifying it does not exist. The given <newvalue> may not be zero.

   delete
       Delete <ref> after verifying it exists with <oldvalue>, if given. If given, <oldvalue> may not be zero.

   verify
       Verify <ref> against <oldvalue> but do not change it. If <oldvalue> zero or missing, the ref must not exist.

   option
       Modify behavior of the next command naming a <ref>. The only valid option is no-deref to avoid dereferencing a symbolic ref.

   If all <ref>s can be locked with matching <oldvalue>s simultaneously, all modifications are performed. Otherwise, no modifications are performed. Note that while each individual <ref> is updated or
   deleted atomically, a concurrent reader may still see a subset of the modifications.

    1. oldsha1 SP newsha1 SP committer LF

       Where "oldsha1" is the 40 character hexadecimal value previously stored in <ref>, "newsha1" is the 40 character hexadecimal value of <newvalue> and "committer" is the committer’s name, email
       address and date in the standard Git committer ident format.

   Optionally with -m:

    1. oldsha1 SP newsha1 SP committer TAB message LF

       Where all fields are as described above and "message" is the value supplied to the -m option.

   An update will fail (without changing <ref>) if the current user is unable to create a new log file, append to the existing log file or does not have committer information available.