Create git branch from web with existing branch + "/" prefix yields 500 and 404 errors (BB-10036)

It seems the "create branch" feature on the web site doesn't properly prevent creating branches that appear as a "subdirectory" of an existing branch.

I made a test repository here: https://bitbucket.org/fluff/bitbucket-test-branch-slash

Steps to reproduce:

1. Create a branch through the web interface, for example "test"
2. Create another branch through the web interface, like "test/another"

Result:

• Step 2 produces a 500 "something went wrong" page (this may be useful in debugging the problem: a3b120c0f7e9461194d4b63a4b8fca30 )
• The invalid branch is still listed in the project's recent activity feed + the dashboard recent activity feed
• Clicking the link to the invalid branch yields a 404 page.
• The branch doesn't actually exist and doesn't appear in the branches page (as expected)