optimize Git plugin’s grename logic by reducing if conditions

[LucasLarson]

Standards checklist:

• The PR title is descriptive.
• The PR doesn't replicate another PR which is already open.
• I have read the contribution guide and followed all the instructions.
• The code follows the code style guide detailed in the wiki.
• The code is mine or it's from somewhere with an MIT-compatible license.
• The code is efficient, to the best of my ability, and does not waste computer resources.
• The code is stable and I have tested it myself, to the best of my abilities.

Changes:

This is an optimization. The function to rename a Git branch using @ujwaldhakal’s grename (#8622, e8609b8) requires both arguments $1 and $2, but the existence of $2 occurs only when $1 exists. Only one check for the existence of $2 (the new name of the branch) is needed.

[ujwaldhakal]

Thanks, it looks good !

[mcornella]

This is more frail ($1 might be empty string) and it's not worth the risk for the null benefits of this optimitzation. Thanks for your contribution though.