git-gui: check browser/blame arguments carefully

git-gui: check browser/blame arguments carefully

git gui offers two related commands, browser and blame, that provide
graphical interfaces driven by git ls-tree and git blame. As such, the
arguments to git-gui need to satisfy those two git commands. But,
git-gui does not assure this leading to confusing or incorrect results.
For instance 'git browser <non-existent path>' shows a blank browser
window rather than error message.

Also, commit 3e45ee1ef2 ("git-gui: Smarter command line parsing for
browser, blame", 2007-05-08) implemented code to allow giving path
before rev on the command line, and unconditionally uses the worktree to
disambiguate. As a result, the following command run in a current
git-gui checkout of the master branch shows the master branch version of
blame.tcl, when none should be shown as that file does not exist in
gitgui-0.6.0.

  git gui blame lib/blame.tcl gitgui-0.6.0

This 'file before rev' feature in git-gui mirrors ideas considered when
git's user interface was very young, but no such feature is documented
for any git command.  Rather than try to fix an idea git itself
rejected, let's just remove this broken and hopefully unused feature.

git-gui browser|blame both accept 'rev' and 'path' as command line
arguments.  rev defaults to 'HEAD' if not given, while path must be
given. path names a directory tree to ls-tree or a file to blame. path
must exist in rev for ls-tree and for blame.  In addition git blame will
include uncommitted changes from the worktree file at 'path' if rev is
not given (thus defaulting to HEAD), but still requires that the file
exists in HEAD.

So, let's clean up the parser to check that the arguments are usable.
- give a full synopsis, including '--' that may be used to separate rev and
  path. (as path is the required final arg, -- gives no extra info)
- explicitly check the number of arguments
- use rev-parse to assure a user supplied rev is valid
- use ls-tree to assure that path exists in rev
- for blame only, with no rev given and a worktree existing, also assure
  that path points to a file in the worktree

With these changes, error messages are thrown by the parser if the path
or rev are not known: no blank or erroneous displays are created. Also,
this avoids accessing the worktree except in the specific use case
supported by blame / git-blame, meaning browser|blame now also work
without a worktree.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>