gomod: update zoekt

Created by: keegancsmith

Includes the following changes:

• 3a0c63c build: SetDefaults uses same defaults as cmd line
• 6a12018 Do not ignore large files.
• f53752c build: move incremental decision into options
• 3a7bd00 all: migrate go-git to new package
• ba74b6d gomod: set minimum version to go 1.13
• 5dd4713 build: factor out build flags
• 4ff72d6 Improve performance for concatened queries
• 9220fb2 web: print name in file view
• 983e877 web: focus on searchbox for '/' keypress
• 77beb0f cmd/zoekt: support -l flag, for compatibility with grep
• b0c18f6 zoekt-mirror-github: add arguments to filter repos on topics criteria

And for completeness, here are the changes from the previous update which didn't include this log:

• 118acdf docker: use go.mod psuedo-version for tag
• 1a9304f ctags: use same command line flags as sourcegraph
• 02b77c3 docker: put .ctags.d into $HOME
• f75c48e bump ctags version to match sourcegraph repo
• d723377 Improve performance for concatened queries
• 6727685 zoekt-webserver: Allow sourcegraph admin proxy to access net/trace