Fix metadata escaping

[KingGorrin]

What?

The build fails when merging a branch.

ERROR: lib/utils/metadata.dart:3:33: Error: Expected ';' after this.
ERROR: const String gitCommitMessage = 'Merge branch 'master' into branch-x';

Why?

The build fails when one of the git variables contains special or escape characters like single quotes or \t.

The variables in the metadata.dart file are generated by scripts during the build. Having special characters in the git variables results in an incorrect variable declaration.

Single quotes are used by gitHub when merging a branch, which will generate the following incorrect variable declartion.

// Incorrect variable declaration
const String gitBranchName = 'Merge branch 'master' into branch-x';
const String gitCommitMessage = 'Char test `~!@#$%^&*\t()_+-={}[]:";'|\<>?,./';

How?

By using raw strings, the escape characters are not interpreted and only single quotes need to be replaced with '"'"r'.

// Correct variable declaration
const String gitBranchName = r'Merge branch '"'"r'master'"'"r' into branch-x';
const String gitCommitMessage = r'Char test `~!@#$%^&*\t()_+-={}[]:";'"'"r'|\<>?,./';