fix: use --fixed-value flag when unsetting git config values

The tryConfigUnsetValue method was passing file paths directly to
`git config --unset`, which treats the value argument as an extended
regular expression. File paths contain `.` characters that would match
any character instead of literal periods, potentially causing incorrect
matches.

Adding the --fixed-value flag ensures the value is treated as a literal
string, fixing credential config cleanup in the v6-style authentication.

src/git-command-manager.ts

@@ -368,7 +368,7 @@ export class GitCommandManager {
     } else {
       args.push(globalConfig ? '--global' : '--local')
     }
-    args.push('--unset', configKey, configValue)
+    args.push('--fixed-value', '--unset', configKey, configValue)
     const output = await this.exec(args, {allowAllExitCodes: true})
     return output.exitCode === 0
   }