Add alias for git pull --prune

[bittner]

Adds a new alias for pulling changes with Git.

Description

From the git-pull man page:

-p, --prune
Before fetching, remove any remote-tracking references that no longer exist on the remote.

Motivation and Context

When working with feature branches it's necessary to clean up branches from time to time. The alias makes this recurring task easier.

How Has This Been Tested?

I have the exact same changes integrated in a local, fresh installation of Bash-it on my developer laptop.

I grepped for "glp" over the current repository HEAD to verify that there are no conflicting aliases.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[bittner]

That's a very simple change. Can this be merged?

[bittner]

On macOS (6+5 =) 11 tests are not run (namely, the tests on install and xterm), that's why the test job fails:

...
ok 64 completion bash-it: enable - provide the todo.txt-cli aliases when todo plugin is enabled with the new location and priority-based name
ok 71 uninstall: verify that the uninstall script exists
...
ok 237 plugins ruby: PATH includes ~/.gem/ruby/bin
ok 243 themes base: battery_percentage should not exist
...
# bats warning: Executed 257 instead of expected 268 tests

On Ubuntu those seem to be run:

...
ok 64 completion bash-it: enable - provide the todo.txt-cli aliases when todo plugin is enabled with the new location and priority-based name
ok 65 install: verify that the install script exists
ok 66 install: run the install script silently
ok 67 install: verify that a backup file is created
ok 68 install: verify that silent and interactive can not be used at the same time
ok 69 install: verify that no-modify-config and append-to-config can not be used at the same time
ok 70 install: verify that the template is appended
ok 71 uninstall: verify that the uninstall script exists
...
ok 237 plugins ruby: PATH includes ~/.gem/ruby/bin
ok 238 plugins xterm: shorten command output
ok 239 plugins xterm: full command output
ok 240 plugins xterm: shorten dirname output
ok 241 plugins xterm: full dirname output
ok 242 plugins xterm: set xterm title
ok 243 themes base: battery_percentage should not exist
...

I can't spot why and why this would be a problem. - Any ideas?