BUG: Allow branches that conflict with file names

[NickCrews]

Before, if I had a file named "benchmarks" and a branch specified in asv.conf.json also named "benchmarks", then when I ran asv publish I would get

$ asv publish
[ 11.11%] · Loading machine info
[ 22.22%] · Getting params, commits, tags and branches
[ 22.22%] ·· Error running /usr/local/bin/git rev-list --first-parent benchmarks (exit status 128)
             STDOUT -------->
             
             STDERR -------->
             fatal: ambiguous argument 'benchmarks': both revision and filename
             Use '--' to separate paths from revisions, like this:
             'git <command> [<revision>...] -- [<file>...]'

Traceback (most recent call last):
  File "/Users/nickcrews/Documents/projects/dedupe/.venv/bin/asv", line 8, in <module>
    sys.exit(main())
....

Tested locally, it works fine now.

[NickCrews]

If it's not too hard, and if you point me to where you want it, I can add a test for this regression. Thanks!

[mattip]

Still missing a test ...

[NickCrews]

If you're familiar with it, could you point me to where exactly you want me to add the test? I can dig around but I'm not at all familiar with this codebase.

[HaoZeke]

Should be good to go now @mattip.

[mattip]

Thanks @NickCrews, @HaoZeke

[NickCrews]

Thank you for finishing this up, I really appreciate it!