[Fix #1022] Fix Fossil detection

[michaelbartnett]

The fossil checkout database is named .fslckout or _FOSSIL_, not
.fossil. Searching for a dominating .fossil file would just find
~/.fossil, which is Fossil's user config database.

I made the changes projectile-project-vcs such that when it searches for a Fossil repo checkout in the current directory or looks for the dominating project file, it tries .fslckout, then _FOSSIL_ for the reasons described above.

@npostavs identified the needed changes in #1022's comment thread.

There were no existing tests for projectile-project-vcs, and figured having Fossil as a test dependency would be no good, so I did not add or change any tests for this.

I did a manual:

1. Created a Fossil repo and checked it out, and ran touch .projectile.

   testfossil1/
     test.fossil
     .fslckout
     .projectile
   

2. Ran fossil add .projectile; fossil commit -m "add projetcile file"; touch test.txt

   testfossil1/
     test.fossil
     .fslckout
     .projectile
     test.txt <- new file
   

3. Verified that projectile-find-file listed .projectile but not test.txt.

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Thanks!

[bbatsov]

👍 Thanks!