Fixes #788.

Apparently this is still run by people so let's tell them to save
some typing.

This option is used to test updating between tags.

As requested in #1199 this updates the documentation accordingly for
that change.

Set the manpage date to the existing one if we're checking for changes.
This avoids the only change being e.g. a new date.

The --fail-if-changed option will cause `brew man`
to return a failing status code if the man files
have changed.

Older versions of Git don't have this flag and we don't want to block
updates for them when there's a (relatively) simple workaround.

some build systems check ac_cv_search_clock_gettime instead of
ac_cv_func_clock_gettime so the former should also be set to "no"

libev.m4 unsets ac_cv_func_clock_gettime, but if ac_have_clock_syscall
is defined, it will leave ac_cv_func_clock_gettime alone:

  http://cvs.schmorp.de/libev/libev.m4?view=markup#l23

We silenced tar in 5e3a26bf. It makes sense to make unzip and unrar
output comparably succinct.

Will only happen once but could be confusing anyway.

intended to be used in combination with --dry-run for a not-so-dry run

the expected file modifications are made, but no git actions are taken

and add explicit version when not preexisting

--version=1.2.3 option to set `version "1.2.3"`

Note that --version=0 will remove a no-longer-needed explicit version

test.m4a contains an AAC LC stream inside an M4A (MPEG-4 Part 14)
container. It is generated by FFmpeg 3.1.3 from test.wav:

    ffmpeg -y -i test.wav -c:a aac -profile:a aac_low -b:a 32k test.m4a

test.m4a facilitates testing utilities or libraries that work with the
AAC codec or the M4A container, e.g., alac.

This is no longer relevant in the bright, new tagged future.

Fixes #1151.

Only run for Homebrew developers so assume they don’t want to switch
back to an old stable branch.

Fixes #1141.

This makes the code easier to follow rather than having to know
HOMEBREW_NO_UPDATE_CLEANUP implies not updating to a tag.

Fixes #1138.